Classic Roast Beef Sandwich with Blue Cheese and Caramelized Onions

Method

1. Heat butter and olive oil in a skillet over medium heat.
2. Add onion slices and cook until softened and golden brown, about 10-12 minutes.
3. Season with salt and pepper.
4. Slice roast beef thinly against the grain.
5. Toast bread slices in a toaster or on a griddle.
6. Layer beef, onions, and blue cheese on bread.
7. If using, drizzle with balsamic vinegar before serving.
8. For whole cuts, check a minimum internal temperature of 63°C (145°F), then rest for at least 3 minutes before serving.