Preheat the oven to 450ºF. Lightly coat a 9x13-inch baking dish with nonstick spray.
Sauté the veggies: In a large nonstick skillet, heat the olive oil over medium-high. Add the diced eggplant, zucchini, salt, and pepper. Cook, stirring occasionally, until the veggies are tender and just beginning to brown (about 4–6 minutes).
Steam the spinach: Add ½ cup of water and the spinach to the skillet. Cover and let it steam for about 3 minutes, stirring once, until wilted.
Make it saucy: Stir in the tomato sauce and let it heat through (1–2 minutes). Remove from heat and mix in the fresh basil.
Layer it up: Arrange the polenta slices in a single layer at the bottom of your baking dish (trim to fit if needed). Spoon the veggie-sauce mixture on top and sprinkle with mozzarella.
Bake for 15 minutes, or until bubbling and the cheese is melted.
Let it rest for 5–10 minutes before slicing and serving. (This helps it firm up and slice beautifully.)