How to Make It
Preheat the oven to 350°. Generously butter a 9-inch round baking dish. In a large heavy skillet, melt the butter in the oil over moderately high heat. Add the onion, bell pepper and potatoes and cook, stirring, for 3 minutes. Stir in the garlic and cook until the vegetables are partially softened but not browned, about 2 minutes. Add the squash and zucchini and cook, stirring, until almost tender, 4 to 5 minutes. Set aside.
In a large bowl, toss together the polenta, flour, Parmesan, thyme, salt and pepper. In a separate bowl, whisk together the milk and eggs, then whisk them into the polenta mixture until combined. Stir the vegetables into the loose batter and spread it in the prepared baking dish. Bake on the middle shelf of the oven for 25 to 30 minutes, or until firm. Sprinkle the Gruyère on top and bake for about 7 minutes, or until golden around the edges. Serve hot or at room temperature.