For the Cheddar Cheese Sauce: Mix grated cheese and cornstarch in a large bowl, toss. Transfer to medium-sized saucepan. Pour in 1 cup evaporated milk and the hot sauce. Cook over low heat, whisking occasionally, until sauce is bubbly and homogenous, about 5 minutes. It will thicken considerably as it warms up. Keep warm, thinning cheese sauce with additional evaporated milk if it becomes too thick. Meanwhile, for the Beef and Buns: Adjust racks to upper middle and lower positions and preheat oven to 300°F. Place roast beef slices in a heatproof dish, cover with aluminum foil, set on the bottom rack, and cook until warmed through, about 5 to 10 minutes. Meanwhile, wrap buns in aluminum foil, set on top rack, and cook until warmed, about 5 minutes. For the Horseradish Sauce: Add sour cream, horseradish, chives, and lemon juice to a medium-sized bowl. Stir until combined. Season with salt and pepper. Smear the cut-side of the bun bottoms with horseradish sauce, and top each with 1/4-pound of the sliced meat. Drizzle with some of the cheddar sauce and then crown with top bun halves. Serve immediately.