The in-form striker put Forest 2-0 up with a header and a penalty in the first half before Ross McCormack halved the deficit with a precise free-kick. The Cottagers turned the game on its head after the break, mounting a 3-2 lead before Michail Antonio drew the hosts level. It was then down to Assombalonga and Jamie Paterson to put the game beyond their visitors.
Stuart Pearce was forced into making three changes to his side in light of injuries sustained in the East Midlands derby.
Chris Cohen, Andy Reid and Jack Hobbs were all ruled out, opening the door for Robert Tesche, Ben Osborn and Danny Fox to start against The Cottagers. Among Pearce’s substitutes were David Vaughan, Jamaal Lascelles and Kelvin Wilson.
Fox reclaimed his place in the centre of defence and was tested for the first time on four minutes. His defensive header cleared the ball only as far as Tim Hoogland, whose well-struck effort was comfortably saved by Karl Darlow.
The keeper was tested again a minute later, this time by McCormack. The former Leeds man struck the ball sweetly from range but, after momentarily letting it slip, Darlow held firm.
Fulham had started the brighter of the two sides but it was Forest who took the lead on nine minutes. Antonio won a free-kick near the left corner flag and Assombalonga rose highest to meet Henri Lansbury’s excellent delivery, nodding the ball into the turf and over the line.
And Antonio was heavily involved as Forest doubled their lead ten minutes later. The winger’s brilliant run took him into the area, where he was brought to a shuddering halt by Shaun Hutchinson. Referee Darren Deadman pointed to the spot and both Hutchinson and the protesting Dan Burn found themselves in his book. Assombalonga brushed the ball to Gabor Kiraly’s right, despite the keeper guessing correctly, to notch his second goal of the evening.
Forest kept up their pressure and had half-chances through Burke and Antonio, the former seeing a shot blocked before the latter clashed heads with Nikolay Bodurov as the pair challenged to meet Chris Burke’s tempting cross from the right.
But the visitors were back in the game on 31 minutes when Hunt fouled Konstantionos Stafylidis in a dangerous position. McCormack curled the ball over the wall and into Darlow’s bottom corner to halve the arrears.
Assombalonga had a half-chance to secure a hat-trick on 37 minutes after neat build up involving Tesche and Eric Lichaj. The American checked on to his right foot before crossing the ball into the middle, where Assombalonga could only nod it wide with Kiraly committed.
Lichaj had to be alert at the other end on 42 minutes, however, as he provided a vital intervention to preserve his side’s lead. McCormack played a lovely through ball to strike partner Hugo Rodallega, who rounded Darlow and looked set to score. But Lichaj scampered back on to the line to divert the ball away from danger and ensure Forest went into the break ahead.
Chris David replaced Hutchinson at the break but Pearce sent his side back out unchanged.
The half began in controversial fashion as Rodallega fell to the turf on his way through on goal. To Fulham’s despair, Mr Deadman and his assistant agreed there had been no contact from Fox and waved play on.
Fulham were nevertheless on top and, after Bodurov had fired a warning shot just over Darlow’s bar from the edge of the box, they scored a deserved equaliser through Rodallega. The Columbian ghosted into the area unmarked to latch on to a deep cross from the left, volleying the ball brilliantly first-time beyond Darlow and into the bottom corner of the net.
Lichaj was replaced by Wilson on 56 minutes, the centre back moving into the middle with Fox replacing Lichaj at left back.
Forest’s new-look defence were put under pressure immediately as Rodallega raced on to another ball over top. Stretching, the striker did brilliantly to send the ball towards goal, forcing Darlow into an unorthadox save to tip it behind.
The Reds had lacked the energy shown in previous performances and it was after lacklustre work by Antonio, who had contributed so much at the other end of the pitch, that the visitors took the lead. The winger was robbed of possession down the left and McCormack was picked out in acres of space on the edge of the area. His shot took a deflection off Mancienne and, with Darlow wrong-footed, the ball bobbled into the net.
Paterson was then introduced in place of Osborn, but still the visitors pressed - McCormack stinging the palms of Darlow from 30 yards.
But on 77 minutes Antonio atoned for his earlier error in quite magnificent style. Mancienne lobbed the ball up to the winger on the edge of the area. He took the ball down and spun his marker in one moment of astonishing skill before finishing neatly beyond Kiraly.
And two minutes later Forest retook the lead in dramatic style. Hunt’s cross from the right was met by Assombalonga, whose header beat Kiraly but not the post. The ball cannoned back across goal and the striker reacted quickest to turn it home for his first Forest hat-trick.
Patrick Roberts, on for Mark Fotheringham, showed promising signs for the visitors as they desperately sought a way back into the match, but on 89 minutes Forest dealt a conclusive blow. Antonio released Paterson down the right and, in familiar style, the winger jinked back inside his defender before rolling the ball under Kiraly for his first goal of the season.
Six minutes of added time were indicated at the end of a scintillating affair, during which Jamaal Lascelles replaced Tesche, but Forest this time held on to their lead comfortably to reclaim top spot and maintain their unbeaten start to the campaign.
Forest: Darlow, Hunt, Mancienne (c), Fox, Lichaj (Wilson 56'), Burke, Tesche (Lascelles 90'), Osborn (Paterson 69'), Antonio, Lansbury, Assombalonga
Subs not used: De Vries, Veldwijk, Vaughan, McLaughlin
Scorer: Assombalonga 9', pen 21', 79' Antonio 77', Paterson 89'
Bookings: Paterson 90', Lascelles 90'
: Kiraly, Bodurov, Hutchinson (David 46'), Burn, Amorebieta, Hoogland, Parker, Fotheringham (Roberts 81'), Stafylidis, McCormack, Rodallega
Subs not used: Bettinelli, Eisfeld, G. Williams, Hyndman, Kavanagh
Scorers: McCormack 31', 65', Rodallega 51'
Bookings: Hutchinson 20', Burn 20', Fotheringham 50', Amorebieta 58', Bodurov 68', Parker 72'
Referee: Darren Deadman
Attendance: 22,572 (452 away)