Slot guided Liverpool to a resounding 3-1 victory over Fonseca's Milan at San Siro, as his team remained calm after a shaky start, storming back after conceding a goal in the opening three minutes.
"To come back so strong and dominant, it was a deserved victory," said Slot, who replaced retired manager Jurgen Klopp at the Merseyside club.
"This is an evening I will remember in the future. Hopefully, many more times like this will come as Liverpool manager."
Ibrahima Konate, Virgil van Dijk and Dominik Szoboszlai scored for Liverpool after Christian Pulisic capitalised on some disorganised defending by the Reds for Milan's lone goal.
A day after Fonseca said his men would have to be "defensively perfect" against Liverpool, they were porous and far from perfect. And it could have been significantly worse with Liverpool enjoying 11 shots on target to the home side's two.
"We played against Liverpool, a great team," Fonseca, who replaced Stefano Pioli at Milan, told Sky. "We started the match well offensively and defensively, then we conceded two goals from set pieces which changed the game.
"When details are missing we can have problems doing what we prepare in training, Liverpool are more of a team than us, we have to work to play without problems for 70/80 minutes."
Asked about conceding almost identical goals from two centre-backs - Konate and Van Dijk - Fonseca said: "For the first goal we had a man on Van Dijk, we had prepared this. He was free, it's true, but the central defenders had to be taken with individual marking."
The two managers have had wildly different starts to their respective leagues this season, with Slot's men winning their first three games before a shock 1-0 loss to Nottingham Forest on Saturday to sit fourth in the Premier League while AC Milan sit 10th in the Serie A table with one win in four games.
