Preheat the oven to 375 degrees.
Heat the oil in a skillet over medium heat. Add the onions, peppers, squash, and garlic. Sprinkle with salt and pepper. Saute for 5 minutes or until just beginning to soften.
Place the softened vegetables in a 9" square baking pan.
In a small bowl combine the milk, mozzarella cheese, and egg. Mix well and pour over top of the vegetables.
Place the tomato slices on top of the vegetables. Sprinkle with the panko breadcrumbs.
Bake for 25-30 minutes or until the breadcrumbs have browned.