Cook the macaroni in a large pot of salted boiling water for 7-8 minutes until al dente; drain and set aside.

Caramelize the onions in a skillet with olive oil for 20-25 minutes, then add balsamic vinegar and cook for an additional 2-3 minutes.

Sauté the mushrooms in the same skillet with remaining olive oil for 5-6 minutes, then combine with onions.

Preheat the oven to 350°F (175°C).

Make the cheese sauce by melting butter, whisking in flour, and gradually adding milk until thickened.

Stir in shredded cheeses, seasoning with salt and pepper.

Combine cooked macaroni with cheese sauce and fold in onion-mushroom mixture.

Transfer to a greased baking dish and sprinkle with panko breadcrumbs.

Bake for 20-25 minutes until golden brown and bubbly.

Let rest for 5 minutes, then garnish with parsley before serving.