1. Place the chiles de árbol in a small bowl with warm water to cover. Let soak until they are soft and pliable, about 30 minutes.
2. Rinse the tuna and blot dry. With a pestle in a mortar or in a small bowl, mash together the garlic, salt, pepper, and chopped cilantro. Coarsely chop two of the soaked chiles de árbol and add them to the garlic mixture with the lime juice. Arrange the tuna steaks in a baking dish just large enough to hold them, and pour the marinade over the fish. Marinate in the refrigerator for 20 minutes, turning once.
3. If you are using mesquite chunks, build a brisk fire. If you’re using a charcoal or gas grill, preheat to high. Toss ½ cup of the mesquite chips on the coals.
4. To make the salsa, grill the tomatoes until the skins are dark and blistered, 8 to 10 minutes, turning with tongs. Thread the onion and garlic onto a skewer and grill until lightly browned, 4 minutes per side. Transfer the tomatoes, onion, and garlic to a plate and let cool. Scrape any really burnt bits off the tomatoes. Drain the remaining chiles de árbol and tear them into pieces. (For a milder salsa, discard the seeds.) Combine the chilies, tomatoes, onion, garlic, cilantro, and lime juice in a blender and puree until smooth. Add salt, pepper, and lime juice to taste; the salsa should be highly seasoned.
5. Oil the grill grate. Toss the remaining 1 cup of mesquite chips (if using) on the charcoal or in the smoker box of a gas grill. Grill the tuna until it’s cooked to taste: about 3 minutes per side for medium-rare. (That’s how I like my tuna.) Warm the tortillas on the grill. (You’ll need about 20 seconds per side.) Serve the tuna with the salsa and tortillas.