Preheat the oven to 375°F and line a baking sheet with parchment paper.
In a mixing bowl, combine the ground beef, breadcrumbs, egg, onion, 1 tsp salt, and ½ tsp black pepper. Mix until evenly blended.
Divide the mixture into four equal portions and shape each into a small loaf. Place the loaves on the prepared baking sheet.
In a small bowl, stir together the ketchup, brown sugar, and Dijon mustard. Spread half the sauce over the tops of the meatloaves.
Bake the meatloaves for 25 minutes. Remove from the oven, add the remaining sauce, and return to the oven for 10 more minutes.
While the meatloaves bake, bring a large pot of water to a boil. Add the cauliflower florets and cook until tender, about 10 minutes.
Drain the cauliflower well, then return it to the pot. Add the minced garlic, butter, milk, ½ tsp salt, and ¼ tsp black pepper.
Mash the cauliflower with a potato masher until smooth and creamy. Serve alongside the finished meatloaves.