2 Chipotle Chiles, in adobo, smashed
4 Garlic Cloves, finely grated
½ cup fresh Lime Juice
½ cup Mezcal
¼ cup Fish Sauce
¼ cup fresh Orange Juice
¼ cup Soy Sauce
2 teaspoons Dried Oregano, preferably Mexican
1 teaspoon Cumin
1 teaspoon kosher Salt, plus more
1½ pounds Skirt Steak
1 large White Onion, halved, sliced ⅓ inch thick
6 tablespoons Olive Oil, divided
Freshly ground Black Pepper
1 medium Green Bell Pepper, ribs and seeds removed, sliced ⅓ inch thick
1 medium Red Bell Pepper, ribs and seeds removed, sliced ⅓ inch thick
Tortillas, Sour Cream, Grated Cheddar, Pico de Gallo, BA’s Best Guacamole, and Roasted Tomato–Cashew Salsa (for serving)
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.