01 -
Combine ketchup, light brown sugar, apple cider vinegar, Worcestershire sauce, garlic powder, and onion powder in a small bowl. Whisk thoroughly for two minutes until smooth and glossy. Let stand at room temperature to allow flavors to meld.
02 -
Melt unsalted butter in a small saucepan over medium heat. Add flour, stirring constantly for one minute until just golden and aromatic. Slowly whisk in cold whole milk to prevent lumps. Simmer gently, stirring for five minutes until thickened. Lower heat and add sharp cheddar cheese, stirring until the sauce is velvety smooth. Remove from heat and keep covered.
03 -
Pour beef broth into a large skillet and bring to a gentle simmer. Separate roast beef slices and add several at a time to the broth. Warm each batch for about one minute, turning with tongs until heated through but still tender. Lift out beef, letting excess broth drip off.
04 -
Place onion bun halves, cut side down, on a dry skillet or under a grill. Toast lightly until the inside is just golden for added texture.
05 -
Arrange bottom halves of toasted onion buns on a work surface. Spread a generous spoonful of signature sauce onto each. Pile warmed roast beef on each bun and drizzle with hot cheddar cheese sauce. Cap with remaining bun halves and serve immediately for best texture and flavor.