Peterborough's club record signing showed exactly why manager Ferguson splashed out £1.2 million to sign him from Watford in the summer, at a time when City head coach Sean O'Driscoll wanted him on loan.
Assombalonga's strike to make it 2-0 early in the second half will live long in the memory. Receiving the ball outside the right hand corner of the box he drilled an unstoppable left-footed shot into the opposite top corner of the net.
Tyrone Barnett had fired Posh in front after 24 minutes when City goalkeeper Frank Fielding failed to hold Craig Alcock's 20-yard drive. And Assombalonga capped a man-of-the-match display with a strong solo run and deft finish two minutes from time.
Ferguson said: "While Tyrone's was a real striker's goal, Britt's first was the big moment of the game because it gave us a cushion.
"From my position in the dug-out I knew it was a goal the moment the ball left his foot. It was an unbelievable strike.
"I thought Britt and Tyrone were tremendous. They are working up a really effective partnership for us..
"Britt's second goal showed the appetite he has for scoring. He could have been content with a free-kick when fouled at the start of his run, but the only thing on his mind was finding the back of the net.
"We knew we needed to buy him because if you sign a striker of his ability on loan and he does well his value soars."