steak sandwiches

INGREDIENTS

• 1 (12-ounce) 1-inch thick New York strip boneless beef top loin steak
• Kosher salt
• Freshly ground black pepper
• Olive oil
• 2 yellow onions, sliced in rings
• 1/2 teaspoon fresh thyme leaves
• Mustard Mayo, recipe follows
• 2 mini focaccia buns, sliced in 1/2 (we used a small ciabatta)
• 1/2 cup baby arugula
• 1/3 cup good mayonnaise
• 2 tablespoons Dijon mustard
• 1 tablespoon whole-grain mustard
• 2 tablespoons sour cream
• 1/8 teaspoon kosher salt