The World's Best Sandwich Recipe — Eat This Not That

Ingredients

• 1 medium onion
• 5 Tbsp unsalted butter
• 1 pound prime rib, cut into 4 steaks
• 2 Tbsp chopped fresh rosemary
• 2 Tbsp chopped fresh thyme
• 2 Tbsp black pepper
• kosher salt
• 2 Tbsp olive oil
• 2 medium ciabatta rolls or hearty sandwich buns, halved
• 2 Tbsp thick seeded mustard
• 1 cup arugula leaves