Spice Blackened Steak Gyro with Pickled Onions

Name: Spice-Blackened Steak Gyro with Pickled Onions
Ingredients: 1 small onion, sliced; 1 1/2 tablespoons red wine vinegar; 1 teaspoon sugar; 1 teaspoon ground coriander; 1 teaspoon onion powder; 1/2 teaspoon peppercorns, cracked with the bottom of a heavy pot; 1 (8-ounce) strip steak; 1 teaspoon vegetable oil, plus oil for brushing pita; 1/2 cup beef stock, preferably reduced-sodium; 1 1/2 teaspoons Worcestershire sauce; 2 pocketless pitas; 4 large escarole leaves, tough spines removed