Whisk chiles, garlic, lime juice, mezcal, fish sauce, orange juice, soy sauce, oregano, cumin, and 1 tsp. salt in a medium bowl. Place steak in a resealable plastic bag and pour in marinade. Seal and turn to coat. Chill at least 1 hour and up to 8 hours.

Prepare a gas grill for 2-zone heat; set one burner at medium-high and one or two burners at high. Alternatively, prepare a charcoal grill for high heat. Grill steak over high heat, turning occasionally, until charred, 2–3 minutes per side for medium-rare. Transfer to a cutting board and let rest 10 minutes. Slice against the grain into ½”-thick strips.

Toss onions and 3 Tbsp. oil in a medium bowl to coat; season with salt and black pepper. (If you have a perforated grill pan, now’s the time to pull it out!) Grill over medium-high heat, tossing occasionally, until charred and tender, 6–10 minutes. Transfer to a large plate.

Toss bell peppers and remaining 3 Tbsp. oil in a medium bowl to coat; season with salt and black pepper. Grill over medium-high heat, tossing occasionally, until charred and tender, 6–10 minutes. Arrange on plate next to onions.

Serve steak and grilled onions and bell peppers with tortillas, sour cream, cheese, pico de gallo, guacamole, and salsa alongside for assembling fajitas.