TO MAKE THE POLENTA: In a very large heavy saucepan, heat the olive oil over very low heat. Add the garlic and sauté, stirring constantly, for about 3 minutes, or until softened. Do not let the garlic burn. Add the chicken stock, water, and salt and bring to a boil. Reduce the heat and, when the liquid is simmering, gradually sprinkle the polenta over in a very slow, thin stream, whisking constantly in the same direction until all the grains have been incorporated and no lumps remain. Reduce the heat to very low. Switch to a wooden paddle and stir every 1 or 2 minutes for 20 to 25 minutes, or until the mixture pulls away from the sides of the pan and the grains of polenta have softened. Stir in the butter. The mixture will be very thick.
RINSE 2 LARGE BAKING SHEETS with cold water and shake them dry. Mound half the polenta on each one. Using a rubber spatula repeatedly dipped in very hot water, spread the polenta evenly in the pans until it is just over ¼ inch thick. Cover the pans with tea towels and allow to rest for 1 hour at room temperature or up to 24 hours in the refrigerator.
TO MAKE THE FILLING: In a large skillet, heat the olive oil over medium heat and sauté the onion for 4 to 5 minutes, stirring, or until it is softened. Add the zucchini and mushrooms and stir for 3 to 4 minutes more, or until the vegetables begin to soften. Season with salt and pepper to taste. Add half the spinach, cover the pan, reduce the heat to low, and cook for 2 minutes. Turn the spinach over so that the wilted leaves are on top and add the remaining spinach. Cook, covered, for 2 minutes more, and turn the spinach. Repeat until all the spinach has wilted. Add the fresh herbs, cover, and cook for 1 minute more. Remove from the heat.
PREHEAT THE OVEN to 375°F. Butter the bottom and sides of a large, deep baking dish, attractive enough to serve at the table, either a 10 × 14 × 9-inch rectangular dish or a 13 × 9-inch oval dish.
CUT THE POLENTA into 2 × 4-inch rectangles. Layer the ingredients in the following order: polenta rectangles, ricotta sprinkled with Parmesan, another layer of polenta, vegetable filling, ricotta sprinkled with fontina, and another layer of polenta. Continue layering until you have used up all the ingredients, finishing with a thin layer of the vegetable filling sprinkled with Parmesan and black pepper to taste. Tap the baking dish gently on the counter to settle the ingredients.
BAKE FOR 35 MINUTES, or until the top is golden brown. Let stand for about 7 minutes. Cut into loosely defined wedges and serve immediately.