Caramelize the Onions:

1. In a large skillet, heat 1 tablespoon olive oil and 1 tablespoon butter over medium heat. Add the thinly sliced onions and cook, stirring occasionally, for 15-20 minutes until the onions are soft and caramelized.
2. Add the balsamic vinegar and cook for an additional 2-3 minutes, allowing the vinegar to reduce and coat the onions.

Cook the Mushrooms:

1. In the same skillet, add the remaining ¼ cup butter and the sliced mushrooms.
2. Sauté for 5 to 7 minutes until the mushrooms are golden and soft.
3. Stir in the minced garlic and cook for another minute until fragrant. Set the mushroom mixture aside with the onions.

Make the Cheese Sauce:

1. In the same skillet (no need to clean it), sprinkle the flour over the remaining butter and mushrooms to create a roux. Stir constantly for 1-2 minutes until the flour is slightly browned.
2. Gradually whisk in the half and half and milk, stirring until the mixture begins to thicken (about 5-7 minutes).
3. Reduce the heat to low and stir in the shredded cheddar cheese and grated Parmesan cheese until melted and smooth.

Combine the Pasta and Sauce:

1. Stir the cooked elbow pasta into the cheese sauce until well coated.
2. Add the caramelized onions and mushrooms, gently folding them into the pasta.

Serve:

1. Serve the mac ‘n’ cheese hot, topped with fresh minced parsley for a pop of color and freshness (optional).