Step 1Wash the spinach well but do not dry. Place in a large pot, cover and cook over high heat until wilted, about 5 minutes. Transfer to a colander to cool, then squeeze out the excess moisture. Chop coarsely and set aside.
Step 2Remove the sausage casings and crumble the meat into a large skillet. Cook over medium heat until just done, about 5 minutes. Transfer to a large bowl.
Step 3Heat the oven to 425 degrees.
Step 4Bring a large pot of water to a rolling boil. Add salt, then the pasta and cook until just barely al dente, about 8 minutes.
Step 5While the pasta cooks, wipe the skillet and add the butter. Melt over medium heat. Add the garlic and cook until softened but not browned. Add the spinach and nutmeg and cook 3 to 4 minutes. Add the stock and cream and bring to a boil.
Step 6Drain the pasta well and place in the bowl with the sausage. Pour in the spinach sauce. Add the peppers and toss until completely blended. Taste and add salt and pepper if needed. Butter a 13-by-9-inch glass baking dish and cover the bottom with 1/3 of the pasta mixture. Strew half the fontina cheese on top. Repeat with the remaining pasta and fontina cheese. Combine the Parmigiano and bread crumbs and sprinkle evenly over top. Bake until the top is crusty and the sauce is bubbly, 10 to 15 minutes.