Preheat oven to 200°C (400°F). Grease a 13-inch (33 cm) x 9-inch (22cm) baking dish with oil spray or butter. Set aside.
Heat oil in a large skillet or slow cooker (choose a big enough pan that fits all the zucchinis and meat, about 6 liters/24 cups).
Add minced onion and cook until fragrant and translucent.
Add ground beef - crumbling the beef is the best to create small pieces of meat, garlic powder, and Italian seasoning. Cook beef for 2 minutes until cooked through.
Stir in zucchini slices (or grated zucchini) and tomato sauce. Cover the saucepan and simmer for 3-4 minutes until the sauce forms bubbles on the side of the pan.
Assembling the casserole
Remove from heat and transfer one layer of zucchini beef mixture into the prepared baking dish.
Add shredded mozzarella and continue layers until the pan is full.
Cover the top of the dish with remaining mozzarella cheese, add pepperoni slices, sliced olives, and green bell pepper. Be creative and add more variety of cheese or meat like ham, chicken, cheddar cheese.
Bake uncovered for 25 minutes until cheese is melted.
Cool baked casserole for 5 minutes before serving and avoid burning yourself.
Divide the whole dish into 6 even portions and serve one portion per meal as a breakfast or dinner.
Cool baked casserole before storing in the fridge, in an airtight container, or freeze in individual portions.
Defrost in the fridge the day before and rewarm in a sauce and under medium heat or in the microwave.