Bring a large pot of lightly salted water to a boil. Cook manicotti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 7 minutes. Drain and cool.

Meanwhile, heat a large skillet over medium-high heat. Add sausage; cook and stir until browned and crumbly, 5 to 7 minutes. Drain excess grease.

Add 2 cups spaghetti sauce, garlic, and Italian seasoning to sausage; simmer for about 10 minutes.

Cut open manicotti tubes; lay flat. Place about 3 tablespoons sausage mixture down middle of flattened tubes; divide and sprinkle 2/3 cup mozzarella cheese on top sausage mixture. Roll up tubes.

Preheat the oven to 375°F (190°C). Lightly coat a 9x13-inch casserole dish with cooking spray.

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. Whisk in Fontina cheese, 1/2 cup mozzarella cheese, and Parmesan cheese until thoroughly blended.

Pour 1/2 cheese sauce into the prepared casserole dish. Place stuffed tubes, seam-sides down, over sauce; pour remaining 1/2 cheese sauce on top. Pour remaining 1 cup spaghetti sauce down middle of casserole.