Easy Creamy Bell Pepper Marinara

¼ cup butter

1 small onion, sliced

½ large red bell pepper, cut into matchstick-sized pieces

1 clove garlic, minced

¼ cup all-purpose flour

1 cup milk

½ cup grated Parmesan cheese

32 ounces tomato sauce (such as Classico® Tomato and Basil Sauce)

¼ teaspoon ground black pepper

½ teaspoon balsamic vinegar

1 dash hot sauce, or to taste

1 ½ cups chopped fresh spinach

Heat butter in a large skillet over medium heat; cook and stir onion, red bell pepper, and garlic until tender, 5 to 10 minutes. Stir in flour until dissolved and thickened, about 5 minutes. Pour in milk, stirring constantly, until well mixed. Add Parmesan cheese; stir until cheese is melted, 5 minutes.

Stir tomato sauce into creamy onion mixture; season with black pepper, balsamic vinegar, and hot sauce. Cook and stir until heated through, 5 to 10 minutes. Sprinkle spinach over sauce to serve.