In large, non–stick skillet, heat oil over medium high heat. Brown turkey until fully cooked, breaking into small pieces with a wooden spoon. Transfer to plate.
In large saucepan, melt butter over medium high heat. Stir in onion and cook until translucent, about 5 minutes. Stir in garlic and cook just until fragrant, but not browned, about one minute. Whisk in flour, and cook until it combines with the flour in a thick paste, about one minute. Continue whisking and slowly pour in milk. Bring to boil and reduce heat to maintain a simmer, whisking often, until sauce is thick, about 10 minutes. Whisk in Dijon, salt and fresh cracked pepper. Whisk in cheddar until smooth. Stir in noodles, reserved turkey with any accumulated juices and diced tomatoes. Pour into a greased 13 x 9 inch (32.5 x 23 cm) baking dish. Sprinkle with crumbs and bake in 350°F (180°C) oven until the sauce is bubbling and top is golden, about 30 minutes.