Preheat oven to 350 degrees F
Combine the egg, onion, beef, milk, and breadcrumbs in a large bowl. Season with salt and pepper and form into a loaf and place in a lightly greased baking dish or simply place it into a loaf pan.
Combine the mustard, brown sugar and ketchup in a separate bowl. Mix well and pour over the meatloaf.
Bake for 60 to 80 minutes at 350 degrees F (175 degrees C).
Serve it with a side of mashed cauliflower or mashed potatoes.