01 - In a mixing bowl, whisk together olive oil, smoked paprika, cumin, chili powder, garlic powder, onion powder, salt, pepper, and lime juice. Add the chicken thighs or flank steak, turning to coat evenly. Allow to marinate for at least 15 minutes at room temperature.
02 - Prepare the charcoal grill by arranging hot coals for direct grilling. Bring the grill to medium-high heat, approximately 400°F to 450°F, and allow the grate to preheat for several minutes.
03 - Place the marinated meat directly over the hot coals. Grill chicken thighs for 4 to 5 minutes per side until the internal temperature reaches 165°F, or flank steak for 2 to 3 minutes per side for medium-rare. Aim for a nicely charred exterior with juicy, cooked-through meat. Transfer to a cutting board and rest for 5 minutes before slicing thinly against the grain.
04 - While the meat rests, lay tortillas directly on the grill grate. Toast each side for 15 to 30 seconds until lightly charred in spots and pliable. Stack and wrap in foil to keep warm.
05 - In a small bowl, combine the chopped tomatoes, red onion, minced jalapeño, cilantro, and lime juice. Season with salt and pepper, adjusting to taste. Set aside to let the flavors meld.
06 - Lay warm tortillas flat and layer with sliced meat, shredded lettuce, diced tomatoes, red onion, cilantro, and avocado slices. Spoon fresh salsa over the top and add crumbled queso fresco or shredded cheese if desired. Serve immediately with lime wedges alongside.