Even though this may be a side, I often eat it as my main because it’s so good!

2 large yellow onions, thinly sliced
2 tablespoons unsalted butter
1 tablespoon olive oil
1 teaspoon sugar
1 teaspoon salt
1/4 teaspoon black pepper
2 cups elbow macaroni
2 cups shredded Gruyère cheese
1 cup shredded Parmesan cheese
2 cups whole milk
2 tablespoons all-purpose flour
1 teaspoon Dijon mustard
1/4 teaspoon nutmeg
1/2 cup breadcrumbs
1 tablespoon chopped fresh thyme