Preheat oven to 375 degrees.

Cook pasta for a few minutes less than directed on package.

While the pasta cooks, in a large oven-safe skillet, olive oil and butter. Melt butter over medium heat over and add in the onions and mushrooms. Cook just until tender. Add in garlic and cook for a minute until fragrant.

Sprinkle flour over the mixture. Whisk together and cook for 1-2 minutes.

Slowly add in the milk while whisking to avoid any clumps. Add in the salt and pepper.

Once combined and thickened reduce the heat to low.

Set aside 1/3 of each of the cheeses. Then add the remaining 2/3 of cheeses to the mixture. Stir together until the sauce is smooth and creamy.

Combine the cooked pasta with the sauce. If your skillet is oven-safe, sprinkle the remaining cheese over the skillet. If your skillet is not oven-safe, transfer the pasta to a large casserole dish (something like a 9 x 13). Top with dish with the the remaining 1/3 cheese that you set aside.

Bake for 30 minutes at 375 or until the cheese is melted and bubbly. Serves 6.