Preheat your oven to 400°F (200°C). Grease a 9x13-inch baking dish with butter or cooking spray.

Heat olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3-4 minutes until translucent.

Add the bell peppers and zucchini to the skillet. Cook for 5-6 minutes until the vegetables soften slightly.

Stir in the minced garlic, cumin, and chili powder. Cook for another minute until fragrant. Add the corn, black beans, and diced tomatoes with their juice. Season with salt and pepper to taste.

Simmer the vegetable mixture for 3-4 minutes. Remove from heat and stir in half of the shredded cheese. Transfer to the prepared baking dish.

In a large bowl, whisk together the cornmeal, flour, baking powder, salt, and sugar.

In a separate bowl, whisk together the milk, eggs, and melted butter. Pour the wet ingredients into the dry ingredients and stir gently until just combined.

Pour the cornbread batter evenly over the vegetable mixture. Sprinkle the remaining cheese over the top.

Bake for 25-30 minutes until the cornbread topping turns golden brown and a toothpick inserted into the center comes out clean.

Let the casserole rest for 5-10 minutes before serving.