Roast Beef and Caramelized Onion Panini
- 3 Tbs. olive oil, plus more for brushing
- 2 large yellow onions, about 1 1/2 lb. total, halved
lengthwise and very thinly sliced
- 2 slices country-style bread, each 1/2 inch thick
- 2 Tbs. mayonnaise
- 2 thin slices roast beef
- Salt and freshly ground pepper, to taste
- 1 slice aged cheddar cheese
- 4 thin slices Granny Smith apple
Uncover the pan, reduce the heat to medium-low and cook, stirring frequently, until the onions are golden brown and sweet, 35 to 45 minutes; do not allow them to burn.
Remove the pan from the heat and let the onions cool to room temperature. Set aside 1/4 cup of the onions for the panini; reserve the rest for another use.
Preheat an electric panini maker according to the manufacturer's instructions.
Brush one side of each bread slice with olive oil. Lay the slices, oiled side down, on a clean work surface. Spread the top of each slice with 1 Tbs. mayonnaise. Place the roast beef on one slice and season with salt and pepper. Top with the 1/4 cup caramelized onions, cheese and apple, and season with salt and pepper. Top with the other bread slice, oiled side up.
Place the sandwich on the preheated panini maker and cook according to the manufacturer's instructions until the bread is golden and the cheese is melted, 3 to 5 minutes.
Transfer the sandwich to a cutting board and cut in half. Serve immediately.