Ingredients
Scale
- 4 tablespoons unsalted butter, divided
- 4 tablespoons olive oil, divided
- 3 medium yellow onions, peeled, halved, and thinly sliced
- 1 teaspoon granulated sugar
- 1⁄2 teaspoon salt
- 1⁄4 teaspoon black pepper
- 1 teaspoon fresh thyme leaves, chopped (or 1/2 teaspoon dried thyme)
- 1⁄4 cup beef broth, dry sherry, or water (for deglazing the pan)
- 4 slices hearty sourdough bread (or any crusty artisan-style bread)
- 6 oz Gruyère cheese, shredded (Swiss or Emmental can be substituted)
- 4 oz sharp white cheddar cheese, shredded
Optional Add-Ons (Protein Boost):
- Shredded or sliced grilled chicken breast
- Thinly sliced roast beef
Instructions
Step 1: Caramelize the Onions
- In a large heavy-bottom skillet (cast iron or stainless steel works best), melt 2 tablespoons of butter and 2 tablespoons of olive oil over medium heat.
- Add the sliced onions and stir to coat them in the fat. Cover the skillet and let the onions cook for 5 minutes to soften.
- Remove the lid and sprinkle the onions with sugar, salt, and pepper. Stir to combine. Reduce heat to medium-low, replace the lid, and cook the onions in 5-minute intervals for about 20 minutes. Stir frequently to prevent sticking and ensure even browning.
- Once the onions have turned a deep golden brown, pour in the beef broth (or water/sherry) to deglaze the pan. Use a wooden spoon to scrape up any flavorful browned bits stuck to the bottom. Cook for 1 more minute, then stir in the thyme.
- Remove the skillet from heat and transfer the onions to a bowl. Wipe the skillet clean with a paper towel—you’ll use it again to build the sandwich.
Step 2: Assemble and Cook the Sandwiches
- In the same skillet, melt the remaining butter and oil over medium-low heat.
- Add the 4 slices of bread and lightly toast one side of each slice—about 2 minutes per side. This adds structure so the sandwich doesn’t go soggy.
- Flip the slices so the toasted side faces up. Divide the shredded cheeses evenly across all 4 slices. Cover the skillet briefly (1–2 minutes) to help the cheese start to melt.
- Once the cheese begins to melt, remove the skillet from the heat.
- On two of the slices, spread a generous layer of caramelized onions. If using grilled chicken or roast beef, layer it on top of the onions now.
- Place the remaining cheese-covered slices on top to close the sandwich, toasted side facing out. Press gently but firmly.
- Return the skillet to medium-low heat. Cook each sandwich for 2–3 minutes per side, pressing slightly with a spatula to ensure even browning, until the outside is crisp and golden and the cheese is completely melted.
- Let the sandwiches rest for 1–2 minutes before slicing in half. Serve warm.
Notes
- Don’t rush the onions: If the heat is too high, they’ll burn instead of caramelizing. Keep it at medium-low and stir regularly.
- Use the right pan: A heavy-bottom skillet distributes heat more evenly, making it easier to control cooking.
- Prep everything before you start: Shred your cheese, slice the bread, and have your deglazing liquid ready. This keeps things smooth and stress-free.
- Watch the bread: Bread can burn quickly if the heat is too high. Keep an eye on it and adjust the heat as needed.
- If the cheese isn’t melting: Covering the skillet helps trap heat. If needed, you can transfer the sandwich to a 300°F (150°C) oven for a few minutes to finish melting.
- Prep Time: 15 minutes
- Cook Time: 30 minutes