Steak and Onion Sandwiches with Horseradish Mayonnaise

Name: Steak and Onion Sandwiches with Horseradish Mayonnaise
Yield: 2 Servings
Ingredients: 3 tablespoons butter, divided; 1 medium red onion, thinly sliced into rounds; 1 12-ounce rib-eye steak, about 3/4 inch thick; 1/4 cup mayonnaise; 2 tablespoons prepared horseradish; 2 kaiser rolls, halved horizontally, lightly toasted; 1 cup arugula