Heat a grill or grill pan over medium-high heat.
Add the mayonnaise, sour cream, jalapeños and pesto to a food processor. Pulse until well combined, then season to taste with hot sauce, salt and pepper, pulsing as needed to combine. Set aside.
Brush the steaks with 1 tablespoon of the oil. Sprinkle all over with salt and pepper. Place the steaks on the grill and cook for 3 to 4 minutes per side for medium rare. Remove to a plate to rest.
Drizzle the cut side of the ciabatta with the remaining 3 tablespoons oil, then grill, cut-side down, until nicely charred and heated through, 1 to 2 minutes. Remove to a board.
Spread the mayonnaise mix on the grilled sides of the bread. Thinly slice the steak against the grain.
Shingle the slices of steak on the bottom half of the ciabatta. Top with the arugula, pickled red onions and more pickled jalapeños, if desired. Place the top on the sandwich. Using a sharp knife, cut the loaf into 4 to 6 servings. Serve with your favorite potato chips. |