For the sandwich spread:
3 tablespoons sour cream
1 tablespoon horseradish
1 16-ounce boneless ribeye steak, or the equivalent
1 red, yellow or sweet onion, sliced into half-moons
12-16 shishito peppers, washed and stemmed
1 tablespoon olive oil
2 ciabatta buns, split lengthwise
Salt and pepper, to taste
Make the sandwich spread: Combine sour cream and horseradish in a small bowl; cover and refrigerate until ready to use.
Preheat a gas or charcoal grill to medium-high. Season steak with salt and pepper on both sides. Grill steak for 5-6 minutes on each side for medium-rare. Remove from heat and let steak rest for 10 minutes before slicing into thin strips cross-wise.
While the steak is cooking, heat olive oil in a grill-proof skillet; add peppers and onion slices. Cook, stirring every couple of minutes, until peppers are toasted on the outside and onions have softened and begun to char at the edges. Remove from heat.
Place split rolls, cut side down onto the grill and let them toast up for about 30 seconds.
To assemble the sandwiches, spread some of the sour cream mixture onto each half of your ciabatta buns. Add steak, onions and peppers, distributing evenly amongst buns. Season with salt and pepper and serve.