The Belgian netted twice in the first half as a rampant Nerazzurri raced into a 4-1 lead at the break, with Roberto Gagliardini and Achraf Hakimi also amongst the goals.
Substitute Lautaro Martinez rifled in a fifth for Inter as they secured a dominant victory, despite Gianluca Caprari’s well-taken brace for the hosts.
Inter came flying out of the traps and were ahead within the opening 30 seconds. Hakimi found acres of space out wide and, having played a one-two with Alexis Sanchez to break into the box, cut the ball back for an easy Lukaku tap in.
Yet the same trio that secured Inter’s early advantage continued to prove a threat and Federico Barba was required to provide well-timed blocks to keep both Lukaku and Sanchez out from Hakimi crosses.
The pressure soon told as Inter doubled their lead in style through Gagliardini. The midfielder met Ashley Young’s cushioned pass from Hakimi’s deep cross to viciously rifle home.
Indeed, the opportunities began to mount and the goalscorers combined to extend the lead on the half hour mark, as Gagliardini pinched a loose clearance to thread Lukaku in for an easy finish.
However, Benevento were gifted a way back moments later as goalkeeper Samir Handanovic inexplicably passed straight to Caprari on the edge of the box, and the forward made no mistake.
Yet the three-goal lead was restored shortly before half-time as Hakimi found the goal his play deserved. A Young cross was missed by Lukaku but slack defending allowed Hakimi to ghost in front of the dallying Gaetano Letizia to prod in at the back post.
Inter were tearing through the Benevento defence with ease and ought to have had a fifth before the break, only for Arturo Vidal to nod wide from close in.
The hosts began the second half brightly and were inches away from a second as Gabriele Moncini raced clear and drilled against the foot of the post, whilst Letizia forced a smart stop with an accurate drive.
At the other end, Gagliardini rattled the crossbar with a half-volley after being teed up by Young, before substitute Martinez found the bottom corner with his first touch after finding space in the box.
Benevento hit back immediately however, with Caprari emphatically securing his brace after meeting Gianluca Lapadula’s cutback in the box, but it proved no more than a consolation.
The winning margin ought to have been extended, but substitute Ivan Perisic somehow fired wide with the goal at his mercy following a rapid counterattack late on.
Deep into stoppage time, Inter again hit the crossbar through a floated Christian Eriksen cross from the left, but the Nerazzurri played the final minutes out with little intention to push forward.