In a large skillet, melt a tablespoon of butter over medium heat. Add thinly sliced onions and cook them slowly, stirring occasionally, for about 30 to 40 minutes until deep 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 to cool slightly.

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

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

Preheat your oven to 350°F (175°C). Transfer the mixture into a greased baking dish, spreading it evenly. Sprinkle breadcrumbs on top and bake for 20 to 25 minutes until golden brown and bubbly.

Let it cool for a few minutes before serving. Pair with a crisp green salad or roasted vegetables.