Perfect Steak Sandwich with Creamy Horseradish Sauce (35 Minutes)

Ingredients

• 1 1/2 lb (680 g) skirt steak or flank steak, trimmed
• 2 tablespoons olive oil, plus more for the onions
• 2 garlic cloves, finely grated
• 1 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 1 teaspoon smoked paprika
• 1/2 teaspoon dried oregano
• 1/2 cup (120 g) mayonnaise
• 1/4 cup (60 g) sour cream
• 2-3 tablespoons prepared horseradish, drained if watery
• 1 teaspoon Dijon mustard
• 1 tablespoon fresh lemon juice
• 1 tablespoon finely chopped chives
• Pinch of kosher salt
• 1 large red onion, sliced into 1/2-inch rounds
• 4 ciabatta rolls, split and toasted
• 2 cups baby arugula
• Optional: 4 slices provolone or Swiss cheese