Whisk chiles, garlic, lime juice, mezcal, fish sauce, orange juice, soy sauce, oregano, cumin, and 1 teaspoon 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 high heat; thoroughly clean grates and brush with oil. Alternatively, prepare a charcoal grill for high heat.
Remove steak from marinade ,letting any excess fall back into the bag, and discard the marinade. 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, bell peppers and 6 Tbsp. oil in a medium bowl to coat; season with salt and black pepper. (If you have a perforated grill pan or griddle, now’s the time to pull it out!)
Grill over high heat, tossing occasionally, until charred and tender, 4–8 minutes. Transfer to a large plate.
Serve steak and grilled onions and bell peppers with tortillas, sour cream, cheese, pico de gallo, guacamole, and salsa alongside for assembling fajitas.
Preheat a grill basket or griddle over high heat until very hot, about 2-3 minutes.
Char tomato, onion, serrano and garlic, turning occasionally, until charred on all sides, about 3 minutes for garlic, 4-5 minutes for serrano, 6-8 for onion and 8-10 for tomato. Transfer to a plate to cool. When cool enough to handle, peel garlic.
Purée tomatoes, onion, chiles, garlic, cilantro and lime juice in a blender on low speed, using a plunger to ensure an even texture, until salsa is almost smooth but some small pieces remain; season with salt and more lime juice if necessary.
Smash avocados with a potato masher or fork in a medium bowl or with a molcajete (a Mexican mortar and pestle) until very coarsely mashed.
Mix in onion, jalapeño, garlic, lime juice, salt, and ¼ cup cilantro.