Combine cumin, coriander, 1½ teaspoons salt, ½ teaspoon pepper, garlic, shallot, lime juice and vegetable oil in a resealable bag or large bowl. Add steak and toss to coat. Set aside for 30 minutes at room temperature or refrigerator up to overnight.
Preheat grill to high heat. Remove steak from marinade and let excess drip off. Grill until seared and cooked to taste, 3-5 minutes per side for medium-rare to medium. Transfer to a cutting board to rest 10 minutes, then slice thinly against the grain.
While steak rests, toss the peppers in a large bowl with olive oil and a pinch of salt and pepper. Transfer to a grill pan or basket and cook for 6-8 minutes, turning occasionally, until blistered and charred. Cut off and discard stems.
To assemble sliders, split a bun and spread the bottom with Challenge Lactose Free Butter. Top with 3-5 slices of steak and 3 shishito peppers. Squeeze lime juice over top, spread more butter on the top bun and close the sandwich. Repeat with remaining sliders and serve.