Preheat your broiler to medium-high. Slice King's Hawaiian rolls horizontally and place cut sides up on a baking sheet. Broil for 1-2 minutes until lightly golden.

Preheat your oven to 350°F (175°C). Coat a baking dish with cooking spray and arrange toasted bottom halves of the rolls, cut side up.

In a small bowl, combine mayonnaise and horseradish. Spread half of this mixture on the bottom halves of the rolls. Layer half of the cheese, roast beef, onions, and remaining cheese.

Spread remaining mayonnaise mixture on top halves of the rolls, and place them on the assembled sliders.

In a small bowl, mix melted butter, Worcestershire sauce, onion powder, garlic powder, and parsley. Brush this mixture over the sliders.

Cover with aluminum foil and bake for 20 minutes. Remove foil and bake for an additional 5-8 minutes until tops are golden brown.

Simmer beef broth with garlic, flour, Worcestershire, thyme, and seasonings in a saucepan until thickened. Strain if desired.

Let sliders rest for a few minutes, slice, and garnish with parsley. Serve warm with au jus for dipping.