Special equipment: A 2- to 3-quart broiler-safe baking dish
Position an oven rack about 5 inches from the broiling element, and preheat the broiler.
Heat 2 tablespoons of the oil in a large high-sided skillet over medium-high heat. While the pan is heating, thinly slice the onion; set aside about 1/3 cup for the salad.
Add the sausage to the pan, and break it up into smaller chunks with a wooden spoon. Stir periodically until it is brown all over, about 5 minutes. Add the remaining onion and the squash. Stir for a minute, then add the tomatoes, 2 tablespoons water, 3/4 teaspoon salt and a few grinds of pepper. Cover partially, and let simmer vigorously, stirring occasionally, until reduced and thick, about 10 minutes.
Meanwhile, slice the polenta into 1/4-inch rounds. Butter the baking dish, and shingle the rounds in one layer. Dot the polenta with the pieces of butter. Place the dish under the broiler, and broil the polenta until it is hot and soft, about 4 minutes.
When the sauce is ready, spread it over the broiled polenta. Top with the fontina and grated Parmesan. Return the casserole to the broiler, and broil until the top is browned and bubbly, about 3 minutes.
Meanwhile, toss the spinach in a large bowl with the reserved onion, the remaining 1 tablespoon olive oil, the balsamic vinegar, and salt and pepper to taste. Serve the casserole with the spinach salad on the side.
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