Our grilled fish sandwiches are a simple twist on the Vietnamese banh mi.
Toss carrots, cucumber, radishes, vinegar, sugar, and 1/2 teaspoon salt in a bowl. Mix mayonnaise and Sriracha in another bowl.
Preheat grill to medium-high. Brush fish with oil; season with salt and pepper. Brush grates with oil. Grill fish until bottom edges turn opaque, about 3 minutes. Flip with 2 large spatulas and cook until opaque in center, about 3 minutes. Remove from grill; cut each fillet in half crosswise. Meanwhile, grill rolls until lightly marked.
Spread mayonnaise mixture on all roll halves. Layer fish and pickled vegetables on bottom halves; drizzle with pickling liquid and top with cilantro. Serve immediately, with more Sriracha.