Heat olive oil in a large non-stick skillet over medium heat.
Add the chopped onion and sauté for 3-4 minutes until translucent.
Add the minced garlic and cook for 1 minute until fragrant.
Add the lean ground beef to the skillet and cook, breaking it apart with a spatula, until browned and no longer pink, about 6-7 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.
Stir in the tomato paste and cook for 1 minute.
Add the canned diced tomatoes (with juices), beef broth, dried oregano, dried basil, black pepper, salt, and crushed red pepper flakes. Stir to combine.
Bring the mixture to a simmer, reduce heat to low, and cook uncovered for 8-10 minutes, stirring occasionally, until the sauce thickens slightly.
Add the chopped fresh spinach and cook for another 2 minutes until wilted.
Taste and adjust seasoning if needed.
Serve warm as a complete low-calorie dinner.