Caramelize the onions by melting a tablespoon of butter in a large skillet over medium heat. Add thinly sliced onions and cook slowly, stirring occasionally, for about 30 to 40 minutes until golden brown and caramelized.

While the onions are caramelizing, bring a pot of salted water to a rolling boil. Add elbow macaroni and cook according to package instructions until al dente, usually around 7-8 minutes. Drain and set aside.

In the same skillet with the caramelized onions, lower the heat and add milk and cream. Gradually stir in the grated Gruyère, cheddar, and mozzarella until melted and smooth.

Fold the cooked pasta into the skillet with the sauce and caramelized onions, stirring gently until well-coated.

Preheat your oven to 350°F (175°C). Transfer the pasta mixture to a greased baking dish, spreading it evenly. Sprinkle a generous layer of breadcrumbs on top.

Bake for 20 to 25 minutes until the top is golden brown and bubbly. Let cool for a few minutes before serving.