Ingredients

• Yellow onions, thinly sliced - 2 large
• Unsalted butter - 2 tbsp
• Olive oil, divided - 1 tbsp + 1 tsp
• Kosher salt, divided - 3/4 tsp
• Black pepper, divided - 1/2 tsp
• Fresh thyme, chopped, divided - 2 tsp
• Shallot, minced - 1 small
• Beef stock - 2 cups
• Sourdough bread slices - 4
• Rosemary butter - 4 tbsp
• Swiss cheese, sliced - 6 oz

Steps

1. Caramelize the onions: Heat a large skillet over medium heat and add {{Olive oil, divided:1 tbsp}} and {{Unsalted butter:2 tbsp}}. Add {{Yellow onions, thinly sliced:2 large}}, season with {{Kosher salt, divided:1/2 tsp}} and {{Black pepper, divided:1/4 tsp}}, and cook, stirring occasionally, until deeply golden and jammy, about 40–45 minutes. Stir in {{Fresh thyme, chopped, divided:1 tsp}} during the last minute of cooking, then remove from heat.
2. Make the au jus: In a saucepan over medium-high heat, add {{Olive oil, divided:1 tsp}}. Add {{Shallot, minced:1 small}} and {{Fresh thyme, chopped, divided:1 tsp}} and cook 1–2 minutes until fragrant. Season with {{Kosher salt, divided:1/4 tsp}} and {{Black pepper, divided:1/4 tsp}}, then pour in {{Beef stock:2 cups}}. Bring to a hard boil for 5–8 minutes to concentrate. Strain and keep warm.
3. Butter the bread: Spread {{Rosemary butter:4 tbsp}} evenly on one side of each {{Sourdough bread slices:4}}.
4. Assemble and grill: Place two slices, butter-side down, on a skillet or griddle over medium heat. Top evenly with {{Swiss cheese, sliced:6 oz}} and a generous layer of the caramelized onions. Cap with remaining bread, butter-side up. Cover and cook 3–4 minutes per side, flipping as needed, until the bread is crisp and the cheese is melted.
5. Serve: Halve the sandwiches and serve hot with the warm au jus on the side for dipping.