Place the onion in a nonmetal measuring cup. Cover with water and a small microwave-safe plate. Microwave on high until the water is boiling, about 5 minutes (or add the onions to a small pot of water on the stove, bring to a boil, then turn off the heat). Let stand for 5 minutes, then drain. Stir in the vinegar and sugar. Chill in the refrigerator for 20 minutes to 3 hours.
Combine the coriander, onion powder, and crushed peppercorns on a plate. Press the steak into the spices, turning to coat. Add the oil to a nonstick skillet and heat over medium heat. Sear the steak until it reaches the desired doneness, about 4 to 7 minutes on each side. Transfer to a cutting board. (The charred bits on the bottom of the pan add to this dish, but if the pan is burned heavily, it can be wiped out carefully, off the heat, with a triple layer of paper towels.) Add the beef stock and Worcestershire sauce to the pan (carefully, because it can splatter) and cook until it is reduced to a quarter of its original volume, about 2 tablespoons, after 3 to 4 minutes. Thinly slice the steak, return to the sauce, and toss. Remove from heat.
To assemble: Brush one side of each pita with a little oil. Heat the pita, one at a time, oiled side down, in a hot skillet until slightly crisp but not brittle, about 1 minute each. Lay each pita on a 12-inch square of foil, crisp side down. Layer half the escarole, sauced steak, and drained pickled onions and 1 teaspoon onion pickle juice in a strip down the center of each pita. Roll the bread and foil together into a cone, using the foil for support and to seal the bottom. Peel the foil down as you eat.