Heat the milk and heavy cream in a medium saucepan over medium heat, being careful not to scorch the bottom. Beat the eggs in a medium bowl until homogenous, then set aside. When the milk mixture begins to simmer, add 1 ladleful to the eggs and whisk to combine. Repeat with 2 more ladlefuls, making sure to whisk thoroughly between each addition. Pour the egg mixture back into the simmering milk and reduce the heat to low. Cook, whisking, until the mixture thickens and coats the back of a spoon, 5 to 7 minutes. Remove from the heat and whisk in the mascarpone and fontina until melted. Transfer to a serving bowl and shave the truffle on top, if using. Serve with salami, garlic bread or your favorite crudites!