Steak Sandwiches

Name: Steak Sandwiches
Cook time: 30 min
Total time: 30 min
Cuisine: American
Ingredients: 1/3 cup mayonnaise; 2 tablespoons smooth Dijon mustard; 1 tablespoon whole-grain mustard; 2 tablespoons sour cream (to make this dairy-free, just use additional mayo); Kosher salt; 1 (16-ounce) 1-inch thick New York strip boneless beef top loin or hangar steak; Freshly ground black pepper; Olive oil, or a neutral high-heat oil such as grapeseed; 2 medium-large yellow onions, sliced in thin rings; 1/2 teaspoon fresh thyme leaves (if desired); 2 large or 4 medium (about 6″) focaccia or ciabatta buns, split and toasted, if you wish; 1 cup baby arugula