Ingredients:

  • 2 large Yellow Onions, thinly sliced
  • 3 Tbsp Unsalted Butter (divided)
  • 1 Tbsp Olive Oil
  • 5 tsp Kosher Salt (divided)
  • 2 Tbsp All-Purpose Flour
  • 1 cup Whole Milk, warmed
  • 1 cup Extra Sharp Cheddar Cheese, grated
  • ¼ tsp English Mustard Powder
  • ½ tsp Black Pepper (divided)
  • 5 lbs 80/20 Ground Beef
  • 1 Tbsp Worcestershire Sauce
  • 1 pack (12 count) Sweet Hawaiian Rolls, left attached
  • 1 tsp Sesame Seeds

Instructions:

  1. Heat the olive oil and 2 Tbsp butter in a heavy skillet over medium-low heat. Add the sliced onions and 1/2 tsp salt.
  2. Cook the onions for 35–40 minutes, stirring occasionally. Maintain low heat until they soften, turn deep golden brown, and reduce significantly in volume. Remove the onions from the pan and set aside.
  3. Prepare the Roux: Melt 2 Tbsp butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute until a smooth paste (roux) forms.
  4. Add Milk: Gradually whisk in the warmed milk, ensuring no lumps form. Bring to a simmer, stirring constantly, until the sauce thickens and coats the back of a spoon (about 3–5 minutes).
  5. Finish the Sauce: Remove the pan from the heat. Stir in the grated cheddar, mustard powder, remaining salt, and pepper until the cheese is completely melted and the sauce is silky smooth. Set aside.
  6. Prepare Patties: Gently mix the ground beef with Worcestershire sauce, 1 tsp salt, and 1/2 tsp pepper. Divide the mixture into 12 equal portions and shape them into patties.
  7. Cook the Patties: Heat the cleaned skillet over medium-high heat. Cook the 12 patties (in batches, if necessary) for 3–4 minutes per side until they have a good sear and are cooked through.
  8. Assembly Prep: Preheat the oven to 190°C (375°F). Keeping the 12 rolls connected, slice the entire block horizontally. Place the bottom half in a prepared 9x13 inch baking dish.
  9. Layer and Drizzle: Arrange the 12 cooked beef patties tightly across the bottom layer of the rolls. Spoon the caramelised onions evenly over the beef. Pour the sharp cheddar sauce generously over the onions.
  10. Close and Brush: Place the top half of the rolls back on. Brush the tops evenly with the 1 Tbsp melted butter and sprinkle with sesame seeds.
  11. Bake: Cover the dish loosely with foil and bake for 10 minutes. Remove the foil and bake for a final 5 minutes until the tops are golden and the cheese sauce is bubbly.
  12. Serve: Let the sliders rest for 5 minutes before slicing along the pre-cut lines and serving immediately.