Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan.
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 ground beef until mixed; press the meat mixture into the prepared loaf pan.
Bake in the preheated oven until the loaf is no longer pink inside and the juices run clear, about 50 minutes. An instant-read thermometer inserted into the center of the loaf should read at least 160 degrees F (70 degrees C).
While the meat loaf is baking, mix together the ketchup, 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.