Caramelize the onions by cooking sliced onions with butter over medium-low heat for 30-40 minutes until golden.

Cook elbow macaroni in boiling water according to package instructions until al dente, about 7-9 minutes, then drain.

In a medium saucepan, combine milk, cream, and cheeses over medium heat, whisk until melted and smooth.

Fold caramelized onions into the cheese sauce and mix in the cooked macaroni until evenly coated.

Transfer mac and cheese mixture to a baking dish, top with breadcrumbs and extra cheese.

Bake in a preheated oven at 350°F (175°C) for 15-20 minutes until bubbly and golden.

Remove from oven, let cool for a few minutes, then serve warm.