Bring a large pot of salted water to a rolling boil. Cook manicotti until tender yet firm to the bite, about 7 minutes.
Meanwhile, heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain excess grease.
Drain manicotti tubes and allow to cool. Add 2 cups spaghetti sauce, garlic, and Italian seasoning to the sausage; simmer for about 10 minutes.
Cut manicotti tubes open and lay flat. Place about 3 tablespoons of the sausage mixture and some mozzarella cheese down the middle. Roll up tubes.
Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch casserole dish.
Melt butter in a medium saucepan over medium heat. Whisk in flour until a smooth paste forms. Slowly pour in cream, whisking to prevent any lumps. Cook until thickened, about 5 minutes. Add fontina, mozzarella, and Parmesan cheeses; whisk thoroughly to blend.
Pour 1/2 of the cheese sauce into the prepared casserole dish. Place stuffed tubes seam-side down over the sauce. Pour remaining cheese sauce on top. Pour remaining spaghetti sauce down the middle of the casserole dish.
Bake in the preheated oven until golden, 20 to 25 minutes.
