Adjust racks to top and middle positions and preheat oven to 425 degrees. Wash and dry produce.

Cut potatoes into ½-inch-thick wedges. Halve, peel, and thinly slice onion. Mince a few slices until you have 2 TBSP (4 TBSP for 4 servings).

Toss potatoes on a baking sheet with a drizzle of oil, salt, and pepper.

Roast on top rack until browned and tender, 20-25 minutes.

Meanwhile, in a large bowl, gently combine beef*, panko, garlic powder, stock concentrate, minced onion, half the ketchup, salt (we used ½ tsp; 1 tsp for 4 servings), and pepper.

Form beef mixture into two 1-inch-tall loaves (four loaves for 4 servings); place on a second baking sheet. Brush tops of meatloaves with remaining ketchup.

Roast on middle rack until cooked through, 15-20 minutes.

While potatoes and meatloaves cook, heat a large drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add sliced onion and cook, stirring occasionally, until browned and softened, 8-10 minutes.

Add 1 tsp sugar (2 tsp for 4 servings) and a splash of water. Cook, adding more splashes of water as necessary, until onion is caramelized and jammy, 2-3 minutes more. Season with salt and pepper.

Turn off heat; transfer onion to a small bowl. Wipe out pan.

Heat 1 TBSP butter in same pan over medium heat. Add bread and toast until golden brown, 2-3 minutes per side (add more butter and work in batches as needed).

While bread toasts, in a second small bowl, combine horseradish sauce, mayonnaise, and half the mustard (all for 4 servings). Set aside.

Lay toasted bread out on a cutting board. Top half the bread slices with caramelized onion; spread remaining bread slices with sauce (save some for serving).

Cut each meatloaf crosswise into four slices; fan out slices over bread topped with caramelized onion. Close sandwiches and cut in half.

Divide sandwiches and potato wedges between plates. Serve with remaining sauce on the side for dipping.