Place the chicken breasts between two sheets of plastic wrap and gently pound to even thickness (about 1/2 inch). In a bowl whisk olive oil, lime juice, smoked paprika, garlic powder, ground cumin, kosher salt and black pepper. Add chicken, turn to coat thoroughly, cover and refrigerate at least 30 minutes and up to 2 hours.

Charbroil

Preheat a grill or grill pan to medium-high (450°F/230°C). Oil the grate lightly. Remove chicken from marinade and let excess drip off. Place breasts on the hot grill and cook 4-6 minutes per side, flipping once, until internal temperature reaches 165°F (74°C) and you have good char marks. In the last 1–2 minutes of cooking, place a slice of pepper jack cheese on each breast to melt. Transfer to a cutting board and rest 5 minutes, then slice if desired to fit the bun.

Garlic Aioli

While the chicken marinates, combine mayonnaise, minced garlic, lemon juice, Dijon mustard, salt and black pepper in a small bowl. Taste and adjust seasoning. Refrigerate until ready to use. (Aioli can be made up to a day ahead.)

Sandwich Build

Prepare buns: slice ciabatta and spread butter on cut sides. Toast cut-side down in a skillet over medium heat or on the grill until golden and crisp, about 1–2 minutes.

Season tomato and avocado: lightly season tomato slices and avocado with the reserved 1/4 tsp salt and 1/8 tsp black pepper.

Assemble: spread about 1 tbs garlic aioli on both bottom and top halves of each toasted ciabatta. On the bottom half place 2 lettuce leaves, one grilled pepper jack-topped chicken breast (or sliced portions), tomato slices, avocado slices, then top with the other ciabatta half. Press gently to compact.

Serve: cut sandwich in half if desired and serve immediately while chicken is warm and cheese is melted.