Heat the olive oil in a large skillet over medium heat.
Add the diced onion and cook for 3-4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet and cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes.
Drain any excess fat from the skillet.
Add the diced red bell pepper, zucchini, and sliced mushrooms to the skillet. Cook for 5 minutes, stirring occasionally, until the vegetables are tender.
Pour in the diced tomatoes with their juice and the gluten-free beef broth.
Stir in the dried oregano, dried basil, salt, black pepper, and crushed red pepper flakes.
Bring the mixture to a simmer and cook for 8-10 minutes, stirring occasionally, until the sauce thickens slightly.
Remove from heat and stir in the chopped fresh parsley.
Serve hot as a main dish.