Whisk garlic, vinegar, mirin, sriracha, and soy sauce in a medium bowl. Whisk in ½ cup oil; season with salt and pepper.
Lightly score steak; season with salt and pepper. Place in a resealable plastic bag with half of dressing. Seal bag and turn to coat. Chill at least 1 hour. Set aside remaining dressing for serving.
Very thinly slice a few sugar snap peas on a diagonal; set aside. Cook remaining peas in a large saucepan of boiling salted water until crisp-tender, about 2 minutes. Drain; transfer to a bowl of ice water and chill until cold, about 4 minutes. Drain, pat dry, and cut in half on a diagonal.
Prepare grill for medium-high heat; lightly oil grates. Remove steak from marinade and grill, turning twice, until medium-rare and lightly charred, 8−10 minutes. Transfer to a cutting board and let rest 10 minutes before thinly slicing against the grain.
Meanwhile, heat remaining 2 Tbsp. oil in a small skillet over medium. Cook peanuts, tossing, until golden brown and shiny, about 3 minutes. Transfer to a small bowl with a slotted spoon and add cayenne; season with salt. Let cool.
Place peanuts in a resealable plastic bag and seal, pressing out air. Pound with a rolling pin to coarsely crush.
Arrange lettuce on a platter; season with salt and pepper and drizzle with half of reserved dressing. Top with steak, cooked peas, cucumbers, and radishes.
Drizzle with remaining dressing and top with peanuts and scallions.