01 - Butterfly each chicken breast by slicing horizontally through the center, creating 4 thinner, even cutlets for faster and more uniform grilling.
02 - In a mixing bowl, whisk together the olive oil, garlic powder, smoked paprika, salt, black pepper, and lemon juice. Add the chicken cutlets and toss to coat evenly. Allow to marinate for at least 10 minutes at room temperature.
03 - Preheat a grill or grill pan over medium-high heat. Place the marinated cutlets on the grill and cook for 4 to 5 minutes per side until fully cooked through with defined char marks and an internal temperature of 165°F.
04 - While the chicken grills, halve and pit the avocado. Scoop the flesh into a small bowl and mash with a fork, seasoning with a pinch of salt and pepper to taste.
05 - In a separate small bowl, combine the mayonnaise and Dijon mustard, stirring until smooth and well blended.
06 - Place the split sandwich rolls or ciabatta buns on the grill cut-side down for 30 to 60 seconds until lightly golden and crisp.
07 - Spread a generous layer of Dijon mayo on the bottom half of each toasted roll. Layer with spinach or lettuce, a grilled chicken cutlet, a slice of cheese if desired, tomato slices, thin rings of red onion, and a hearty spoonful of mashed avocado. Cap with the top half of the roll.
08 - Serve the sandwiches immediately while the chicken is still warm and the rolls are crisp.