In a large skillet, heat 1
tablespoon of the olive oil over medium-high heat and
saute the sausage until golden brown, stirring to break
up the pieces, about 6 minutes. Add half of the onions
and the eggplant, season with 3/4 teaspoon salt and 1/4
teaspoon crushed red pepper, and cook for 4 to 6
minutes, or until the vegetables are lightly caramelized
and very soft. Add 1 tablespoon garlic and cook for 1
minute. Remove from the heat and transfer to a large
bowl to cool.
When the mixture has cooled to room temperature, stir
in the ricotta or mascarpone cheese, spinach, 1/2 cup
Parmesan cheese, and sugar. Stir to combine well.
Lightly grease a deep 9 by 13-inch casserole or
lasagna pan and set aside.
Bring a large pot of salted water to a boil and cook
the pasta shells until just al dente, 10 to 12 minutes.
Drain and rinse under cold running water. Pat shells
dry. Stuff the pasta shells with the sausage-eggplant
mixture and place in the prepared casserole dish.
In a large saucepan, heat the remaining tablespoon of
olive oil over medium high heat and saute the remaining
chopped onions until translucent, about 4 minutes. Add
the remaining tablespoon of garlic and cook until
fragrant, 1 to 2 minutes. Add the crushed tomatoes,
remaining 3/4 teaspoon salt, and remaining 1/2 teaspoon
crushed red pepper and cook for 5 minutes. Add the heavy
cream and cook until the sauce is reduced in volume by
about 1/3, about 20 minutes.
Preheat the oven to 350 degrees F.
Add the basil to the sauce and pour the sauce over
the shells in the casserole. Sprinkle with the fontina
cheese and the remaining Parmesan. Cover the casserole
tightly with aluminum foil and bake for 45 minutes.
Remove the foil and continue baking until the sauce is
lightly browned in spots and bubbly around the edges,
about 15 minutes longer. Let stand for 10 minutes before