Heat olive oil in a large skillet over medium heat.
Add diced onion and cook 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 apart with a spoon, until browned and cooked through, about 6-8 minutes.
Drain excess fat from the skillet if necessary.
Add diced zucchinis to the skillet and cook for 5 minutes until they start to soften.
Stir in the drained diced tomatoes, dried oregano, dried basil, salt, black pepper, and crushed red pepper flakes.
Cook for an additional 4-5 minutes, stirring occasionally, until the zucchini is tender and flavors are combined.
Sprinkle shredded mozzarella cheese evenly over the top of the mixture.
Cover the skillet with a lid and cook for 2-3 minutes until the cheese melts.
Remove from heat and serve warm.