Heat the olive oil in a large skillet over medium-high heat until shimmering.

Add the ground beef and cook until browned, breaking it into small crumbles as it cooks.

Stir in the onion and bell pepper and sauté until softened and fragrant.

Add the garlic and cook briefly until aromatic.

Mix in the marinara sauce, tomato paste, red wine vinegar, Italian seasoning, basil, salt, black pepper, and crushed red pepper flakes.

Simmer the mixture for several minutes until slightly thickened and well combined.

Arrange half of the zucchini slices in an even layer in the prepared baking dish.

Spoon half of the meat sauce over the zucchini and spread evenly.

Repeat with the remaining zucchini and meat sauce to create a second layer.

Sprinkle the cheddar and mozzarella evenly across the top.

Cover loosely with foil and bake until the zucchini begins to soften.

Remove the foil and continue baking until the cheese is melted, bubbly, and lightly golden.

Allow the casserole to rest for several minutes before serving.

Garnish with fresh basil just before serving for a burst of freshness.