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Special equipment: a 2- to 3-quart broiler-safe baking dish
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Position an oven rack about 5 inches from the broiling element, and preheat the broiler.
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Heat 2 tablespoons of the oil in a large high-sided skillet over medium-high heat.
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While the pan is heating, thinly slice the onion; set aside about 1/3 cup for the salad.
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Add the sausage to the pan, and break it up into smaller chunks with a wooden spoon.
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Stir periodically until it is brown all over, about 5 minutes.
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Add the remaining onion and the squash.
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Stir for a minute, then add the tomatoes, 2 tablespoons water, 3/4 teaspoon salt and a few grinds of pepper.
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Cover partially, and let simmer vigorously, stirring occasionally, until reduced and thick, about 10 minutes.
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Meanwhile, slice the polenta into 1/4-inch rounds.
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Butter the baking dish, and shingle the rounds in one layer.
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Dot the polenta with the pieces of butter.
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Place the dish under the broiler, and broil the polenta until it is hot and soft, about 4 minutes.
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When the sauce is ready, spread it over the broiled polenta.
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Top with the fontina and grated Parmesan.
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Return the casserole to the broiler, and broil until the top is browned and bubbly, about 3 minutes.
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17
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.
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18
Serve the casserole with the spinach salad on the side.
