Start to finish: 30 minutes
3 tablespoons butter, softened
1 tablespoon olive oil
1 onion, thinly sliced
2 slices whole-wheat or rye bread
2 ounces deli roast beef, sliced
2 ounce Gruyère cheese, shredded
Heat a nonstick skillet over medium heat. Add in 2 tablespoons butter and olive oil.
Once butter has melted, add in sliced onions and toss to coat. Sauté until translucent, then reduce heat to medium-low. Cook, stirring frequently, until soft and caramelized, about 15–20 minutes. Set aside.
Spread remaining tablespoon of butter on one side of each slice of bread. Layer 2 tablespoons caramelized onions, roast beef and Gruyère atop the opposite side of one slice and top with the other slice, buttered side up.
Place sandwich in skillet and toast until browned and cheese is melted, about 3–4 minutes per side. Slice and serve immediately.