1. Preheat oven to 350 degrees. Place potatoes in a large saucepan; add enough cold water to cover by about 2 inches. Bring to a boil over medium-high heat. Cook until tender, about 20 minutes. Transfer to a colander, and drain; return potatoes to pan.

2. Mash potatoes with a fork or potato masher until light and fluffy. Add butter, cream, 1/2 teaspoon salt, and a pinch of pepper; stir until combined, and set aside.

3. Combine ground beef, onion, mustard, egg, breadcrumbs, 1/4 cup ketchup, 2 teaspoons salt, and 1/2 teaspoon pepper in a large bowl. Place half the mixture on a rimmed baking sheet, and form into an 11-by-6-inch loaf. Place 1 1/2 cups of mashed potatoes on top of meat mixture, leaving a 1-inch border. Place the remaining meat mixture on top; form into a loaf, encasing the potato mixture. Bake 30 minutes, then brush remaining 2 tablespoons ketchup over meat loaf. Continue baking meat loaf until firm to the touch, about 20 minutes more. Serve immediately.