Prepare Au Jus - In a medium saucepan, heat 3 cups prepared au jus over medium-low heat or make your own by bringing 3 cups of water to a boil. Reduce heat to low and whisk in 4 bullion cubes or 4 tsp beef bouillon concentrate until dissolved. Whisk in 1 tsp soy sauce, ¼ tsp garlic powder. Salt and pepper to taste.
Prepare Roast Beef - Buy rare to medium rare thin-sliced roast beef from the market or make your own by roasting 3-4 pounds beef roast (eye of round) in a 425°F (220°C) oven for under an hour. First rubbing meat all over with ¼ cup olive oil, kosher salt, and black pepper and then roasting until an instant read thermometer registers 130°F (55°C), about 40 to 45 minutes. Let it rest for 15 minutes then slice 2 pounds super thin. Add sliced roast beef to the warm au jus.
Prepare Cornstarch Glaze - Add ½ cup cold water and stir 1 tbsp cornstarch in a small saucepan over medium-high heat. Bring to a gentle boil and then reduce heat to low and cook, stirring, until mixture thickens and becomes translucent. Remove from and let cool.
Prepare Rolls - Preheat oven to 300°F (150°C). Place Kaiser or hard rolls on a baking sheet, top-side up. Brush tops with cornstarch glaze and sprinkle with coarse salt and caraway seeds. Bake until rolls get crusty and the salt and caraway seeds begin to stick, a few minutes. Remove from oven and slice rolls in half.
Assemble Sandwich - Divide au jus warmed beef on the roll bottoms. Spoon a little horseradish over the meat and cover with the seeded top. Serve warm.