Heat olive oil in a large skillet over medium heat. Add diced onion and sauté for 3-4 minutes until softened.
Add minced garlic and ground beef to the skillet. Cook, breaking up the beef with a spoon, until browned and cooked through, about 5-7 minutes.
Stir in sliced bell pepper and zucchini. Cook for an additional 3-4 minutes until vegetables are tender.
Add cherry tomatoes, oregano, basil, salt, black pepper, and red pepper flakes (if using). Stir well and cook for another 2-3 minutes until tomatoes are slightly softened.
Remove from heat and sprinkle crumbled feta cheese on top. Garnish with chopped parsley before serving.
Serve immediately with rice, quinoa, or pita bread for a complete meal.