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 cook for 1 minute until fragrant.
Add ground beef to the skillet and cook, breaking it up with a spoon, until browned and cooked through, about 6-7 minutes.
Drain excess fat from the skillet if necessary.
Add diced zucchini, dried Italian seasoning, salt, black pepper, and crushed red pepper flakes to the skillet.
Cook, stirring occasionally, until zucchini is tender but still slightly crisp, about 5 minutes.
Sprinkle shredded mozzarella cheese evenly over the skillet mixture.
Cover the skillet with a lid and cook for 2-3 minutes until the cheese melts.
Remove from heat and garnish with chopped fresh parsley.
Serve warm.