Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet. Cook, breaking it up with a spoon, until browned and cooked through, about 7-8 minutes. Drain any excess fat.
Stir in chili powder, ground cumin, smoked paprika, crushed red pepper flakes, salt, and black pepper. Cook for 1 minute to toast the spices.
Add the diced tomatoes and beef broth to the skillet. Stir to combine and simmer for 5 minutes until slightly thickened. Remove from heat.
In a large mixing bowl, combine the sliced zucchini and the cooked ground beef mixture. Toss gently to combine.
Transfer the mixture to a greased 9×13-inch baking dish, spreading evenly.
In a small bowl, mix the breadcrumbs with the Parmesan cheese.
Sprinkle the shredded cheddar cheese evenly over the casserole, followed by the mozzarella cheese, then the breadcrumb and Parmesan mixture on top.
Bake uncovered for 30 minutes until the zucchini is tender and the cheese topping is golden and bubbly.
Remove from the oven and let rest for 5 minutes.
Sprinkle chopped fresh parsley over the casserole before serving.