Heat the oven to 375°F. Cook onion, carrot and celery in oil in a wide saucepan over medium heat for 7 to 8 minutes until softened.
Add turkey and cook for 7 to 9 minutes, breaking into small crumbles, until it reaches 165°F. Stir in garlic for 30 seconds.
Add tomatoes, tomato sauce, Italian seasoning, sauce salt and pepper and simmer for 10 minutes. Meanwhile, boil mostaccioli in water with the tablespoon salt until 3 minutes short of the package time and drain.
Fold pasta and fontina into the sauce and transfer to a deep 9-by-13-inch baking dish. Scatter mozzarella over the top and cover with foil.
Bake for 25 minutes, then uncover for 8 to 10 minutes until the center reaches 165°F and the top is lightly browned. Rest for 10 minutes before serving.