In a large skillet, heat olive oil over medium heat. Add diced onions and garlic, cooking until fragrant. Then, add ground beef and cook until browned. Drain any excess grease.
Add the zucchini
Stir in sliced zucchini, diced tomatoes, Italian seasoning, salt, black pepper, and red pepper flakes. Let it simmer for 5 minutes until the zucchini softens slightly.
Mix in sour cream
Remove the skillet from heat and stir in the sour cream until evenly combined.
Assemble the casserole
Transfer the beef and zucchini mixture into the greased baking dish. Sprinkle cheddar and mozzarella cheese evenly on top.
Bake
Place in the preheated oven and bake for 20 minutes, or until the cheese is melted and bubbly.
Serve
Let the casserole rest for a few minutes before serving. Enjoy your delicious ground beef zucchini dish!