Add the olive oil, eggplant, zucchini, summer squash, bell peppers, onion, mushroom, garlic, Italian seasoning, tomatoes, salt, and pepper to the slow cooker. For a more traditional ratatouille layer the vegetables.
To make the polenta rounds, preheat the oven to 400 degrees and slice the polenta in 3/4 inch rounds. Season with salt and pepper. Place on a prepared baking sheet and cook for 10 minutes, flipping halfway through.