Pasta with Onions and Cheese (Baked)

6 Tbsp. butter, divided
3 slices bacon, diced
1 cup yellow onion, finely chopped
1 tsp. red pepper flakes
2 small cloves garlic, minced
3 Tbsp. flour, unbleached
3 cups whole milk
3 cups sharp cheddar cheese, coarsely grated and divided
1 cup Parmesan cheese, grated
4 oz Mascarpone or cream cheese
¾ cup panko (Japanese Breadcrumbs)
½ cup fresh parsley, chopped
1 lb penne or other short pasta

Melt 1 Tbsp. of butter in large deep skillet over medium-high heat. Add bacon and cook until crisp. Use a slotted spoon to remove bacon to paper towels to drain. Remove all but about 3 tablespoons of fat from the pan. Add onion and sauté until tender, about 5 minutes. Add red pepper flakes and garlic, stir for 1 minute. Stir in 2 Tbsp. butter, allow to melt, and then add the flour and stir for a minute. Gradually whisk in 3 cups of milk; simmer until thick enough to coat a spoon, stirring frequently, about 5 minutes. Remove from heat and whisk in 2¼ cups of cheddar and all of the Parmesan cheese and marscapone. Return bacon to sauce and season with salt and pepper.

Melt 3 Tbsp. butter in large nonstick skillet over medium heat. Add panko and stir until light golden brown, about 5 minutes. Remove from heat, stir in parsley.

Preheat oven to 375°F (190°C). Lighly butter a 13x9x2" glass baking dish or similarly sized gratin dish.

Cook pasta in large pot of boiling water until very al dente. Drain well. Toss pasta and cheese sauce and season again with salt and pepper, if needed.

Transfer mixture to prepared baking dish and top with remaining cheese and breadcrumbs.

Bake until hot and topping is golden brown, about 30 minutes.

Let sit for 5 minutes before serving.