If you made caramelized onions* in your slow cooker earlier in the week, substitute a few tablespoons of those for the onion listed here. You can save any extra horseradish cheese sauce in the refrigerator for a few days. Makes 2 sandwiches.
1 gigantic or 2 medium onions, thinly sliced*
2 Tbsp olive oil + extra for brushing the panini
1/2 cup mayonnaise (I use Hellmanns)
1/4 cup grated cheese (Cheddar is great)
2 tsp prepared horseradish
Large pinch of fresh black pepper
4 slices of bread
8 slices of roast beef
In a small nonstick frying pan over low heat, combine the onions and 2 tablespoons of olive oil. Cook, stirring occasionally, for 6-7 minutes, until the onions are soft and lightly browned. Remove from heat and set aside.
Stir together the mayonnaise, grated cheese, horseradish and black pepper in a small heat-proof bowl. Microwave for 30 seconds, until the cheese melts. Whisk together until smooth, and set aside.
Heat a panini press (or grill pan), while you assemble the sandwiches. Brush one slice of bread with olive oil, and place it oiled-side down on a plate. Top with a smear of cheese sauce, 2 slices of roast beef, a tablespoon or so of caramelized onions. Smear another slice of bread with cheese sauce, and place sauce-side down on the sandwich. Paint the top with olive oil. Repeat to complete the second sandwich.
Place both sandwiches on your panini press, and cook until the bread is nicely toasted. Cut and serve.