First, marinate the steak: Pat it dry and generously season both sides with salt and pepper. Set aside. For the marinade, mix the lime juice, mezcal, garlic, and serrano chiles in a large bowl. Whisk to combine. Submerge the steak into the marinade and refrigerate covered in a bowl or in a plastic bag for an hour. While steak is marinating, start on the salsa molcajete and avocado crema.
Make the salsa molcajete: Heat grill to medium-high. Place tomatoes and three types of chiles on a large tray; drizzle with olive oil and salt. Toss well. Place vegetables on grill until soft and charred, about 8-10 minutes, grilling both sides. Once vegetables are grilled with even marks, remove from grill and let cool. Remove stems and peel dark skin from tomatoes and chiles.
To grind ingredients in molcajete, start by adding 1⁄2 tsp of coarse salt. Add the peppers one by one and grind down to a coarse consistency. Add the tomatoes one by one and grind. Season mixture with salt and pepper and add lime juice. If you do not have a molcajete, you can use a food processor. Place all ingredients in at the same time and pulse for a chunky consistency.
Make the avocado crema: Place all items in a food processor and blend until smooth. Adjust salt and pepper as needed.
Remove the steak from the refrigerator and let it sit out for about 30 minutes to bring it to room temperature. Meanwhile, heat the grill to a high heat. Remove steak from marinade, shaking off excess, and place over the hottest part of the grill for about 3-4 minutes or until an exterior crust forms. Flip the meat over and continue cooking for about 3 more minutes or until desired degree of doneness. Transfer the steak to a cutting board and let it rest for 5 minutes. Slice the meat thin against the grain.
While the steak rests, place tortillas on the grill until lightly charred and still soft—about 30 seconds each side. Remove the tortillas from the grill and envelope them between halves of a clean, soft dish towel, then cover them to keep warm.
To assemble the tacos: Place all the accouterments on the table (like avocado crema and fresh cilantro) and let guests assemble their own.