Set up two shallow bowls. In one bowl, beat together egg and milk. Pour breadcrumbs in the other bowl, and set both near stove.

Heat olive oil in a large skillet over medium-high. Dip eggplant slices in egg, then dredge in breadcrumbs, and place in skillet. Cook until golden on both sides, then remove to a 9x13 baking dish, arranging in a single layer. (Squish them in if you need to.)

Reduce skillet to medium, and add onions. Cook until translucent, then add tomatoes and zucchini. Cook until tomatoes have broken down a bit, about 5 minutes. Season with salt and pepper.

Preheat broiler.

Pour tomato mixture over the eggplant. Tear mozzarella into pieces and scatter over the top. Cook under the broiler until cheese is melted and bubbly.