Makes 10 mini sandwiches
1 1/2 pounds flank steak
1 tablespoon olive oil
1 teaspoon kosher salt
1 teaspoon black pepper
8 -10mini ciabatta rolls
1/2 cup red onion, sliced into 1/4 inch rings
4 ounces fresh spinach
1/4 cup plain Greek yogurt
1 tablespoon whole-grain mustard
1 teaspoon horseradish, finely grated
1 teaspoon lemon juice
1 teaspoon smoked paprika
Season steak with olive oil, salt and pepper.
Sear steak on one side for 2-3 minutes; turn and continue cooking until internal temperature reaches 135 (medium rare) or desired doneness.
Slice ciabatta rolls in half, toast on the grill until crunchy, 3-5 minutes.
Combine yogurt, mustard, horseradish, lemon juice, paprika and salt.
Slice steak into thin slices against the grain of the meat.
Coat the top and bottom of the roll with horseradish sauce.
Assemble sandwiches by layering steak, onion, and spinach evenly.