In a large skillet, melt 2 tablespoons of butter over low heat. Add 3 sliced yellow onions and cook until deeply golden brown, about 25-30 minutes.

Bring a large pot of salted water to a boil. Add 2 cups of elbow macaroni and cook according to package instructions until al dente, about 7-10 minutes. Drain and set aside.

Stir in 1 cup of vegetable broth into the caramelized onions and let it simmer for about 5 minutes. Then whisk in 1 cup of milk and 1 cup of heavy cream over medium heat.

Gradually add 1 cup each of Gruyère, cheddar, and mozzarella cheese, stirring until melted and smooth.

Remove from heat and fold the cooked macaroni into the cheese sauce. Optionally, stir in fresh thyme.

Transfer to a greased 9x13 inch baking dish, evenly spread, and sprinkle with breadcrumbs and extra cheese.

Preheat the oven to 350°F (175°C). Bake for 20–25 minutes, until golden brown and bubbly.

Let it cool for 5-10 minutes before serving. Garnish with fresh thyme leaves if desired.