In a large skillet over medium heat, sauté the diced onion and bell pepper for about 5 minutes until softened. Add the minced garlic and cook for an additional minute until fragrant.
In a bowl, combine the sliced zucchinis, sautéed vegetables, marinara sauce, Italian seasoning, salt, and pepper. Mix until the zucchini is well-coated.
In a 9x13 inch baking dish, spread half of the zucchini mixture evenly on the bottom.
Sprinkle half of the mozzarella cheese on top of the zucchini layer.
Layer the remaining zucchini mixture over the cheese. Then, sprinkle the remaining mozzarella cheese and finish with grated Parmesan and sliced black olives on top. If you like a bit of heat, sprinkle the red pepper flakes as well.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is melted and bubbly and the top is slightly golden.
Allow to cool for 5 minutes before slicing.
Garnish with fresh basil leaves before serving if desired.
Notes
Feel free to add your favorite toppings or adjust the spices to your taste.