Step 1

In a large oven-safe skillet over medium heat, melt olive oil and 1 tablespoon butter until pan is shimmery. Add onions and let cook, stirring occasionally until onions are a deep caramelized color, about 15-20 minutes.

Step 2

Add mushrooms to pan and saute an additional 3-4 minutes or until mushrooms are softened. Add garlic and balsamic and saute 1 minute until fragrant. Scrape onion/mushroom mixture into a bowl and set aside.

Step 3

In same skillet over medium heat, melt remaining 1/4 cup butter. Whisk in flour until completely smooth and let cook 1 minute until golden brown. Slowly whisk in half and half and milk until mixture is smooth. Bring to a simmer over medium heat until sauce is bubbly and thickened.

Step 4

Remove sauce from heat and whisk in cheeses until melted. Add cooked pasta and onion/mushroom mixture and stir until evenly combined. Season with salt and pepper to taste.

Step 5

Bake mac 'n' cheese in oven-safe skillet or a greased 9x13 baking dish at 375F 15-20 minutes or until bubbly and golden brown. Serve hot and enjoy!