Ingredients:

  • 2 large Yellow Onions, thinly sliced
  • 30 g (2 tbsp) Unsalted Butter
  • 15 ml (1 tbsp) Olive Oil
  • 5 g (1/4 tsp) Kosher Salt
  • 5 ml (1 tsp) Balsamic Vinegar (optional)
  • 120 ml (1/2 cup) Crème Fraîche or Sour Cream
  • 15 ml (1 tbsp) Prepared White Horseradish
  • 3 g (1 tsp) Fresh Chives, minced
  • Pinch of Black Pepper
  • 340 g (12 oz) Leftover Prime Rib, sliced thinly
  • 8 slices Sourdough or Texas Toast Bread (thick-cut)
  • 240 g (8 oz) Gruyère or Aged White Cheddar Cheese, grated
  • 60 g (4 tbsp) Unsalted Butter, softened (for grilling)

Instructions:

  1. Melt the butter and oil in a large skillet over medium-low heat. Add the sliced onions and salt. Stir well to coat. Cook slowly for 20–25 minutes, stirring occasionally, until the onions are deep golden brown, soft, and sweet. Stir in the balsamic vinegar (if using) during the last minute and set aside.
  2. Prepare the Horseradish Cream: In a small bowl, combine the crème fraîche/sour cream, prepared horseradish, chives, and pepper. Mix until smooth. Taste and adjust seasoning as needed.
  3. Gently warm the sliced prime rib in a separate skillet over very low heat for 1–2 minutes, or until just warmed through. Avoid overheating.
  4. Assemble the Sandwiches: Lay out the 8 slices of bread. Spread the Horseradish Cream lightly on the inside of all 8 slices.
  5. Layer the Ingredients: On four slices of bread, evenly layer one quarter of the grated cheese, followed by a generous layer of caramelized onions. Layer the warmed prime rib slices on top of the onions, and finish with the remaining cheese. Top with the remaining four slices of bread.
  6. Butter the Exterior: Generously spread softened butter across the exterior surfaces of all 8 sandwich sides.
  7. Cook the Grilled Cheese: Place the sandwiches on a griddle or large skillet over medium heat. Grill for 3–4 minutes per side until the bread is a deep, golden brown and crunchy, and the cheese is fully melted and gooey. Press gently with a spatula only once per side.
  8. Rest and Serve: Remove from the pan, slice in half diagonally, and serve immediately.