In a bowl, lightly mix the beaten eggs, milk, bread crumbs, onion, salt, black pepper and sage together until thoroughly combined. Gently work in the mince until mixed; press the mince mixture into the prepared loaf tin.
Bake in the preheated oven until the loaf is no longer pink inside and the juices run clear, about 50 minutes.
While the meat loaf is baking, mix together the tomato sauce, brown sugar and dry mustard powder in a bowl until the sugar has dissolved. When loaf is done, pull from the oven and spread topping over the loaf. Return to oven, and bake until the topping is set, about 10 more minutes.