Cook and crumble the ground beef in a large deep sized skillet, or dutch oven, over medium-high heat. Drain any excess grease, if necessary.
Add the onion, bell pepper, and garlic to the pan. Continue cooking for an additional 3-4 minutes, until the vegetables begin to soften. Stir in the crushed tomatoes, tomato puree, and tomato sauce.
Add the oregano, basil, salt, black pepper, and red pepper flakes to the sauce along with the mushrooms. Stir to combine and bring to a simmer.
Reduce heat to maintain a slow simmer and continue cooking, stirring occasionally, for about an hour. Taste the sauce and adjust the seasonings as needed. Sprinkle with fresh parsley and Parmesan before serving, if desired.