01 - Butterfly the chicken breasts by slicing them horizontally, creating 4 thinner cutlets for even cooking.
02 - In a bowl, combine olive oil, garlic powder, smoked paprika, salt, pepper, and lemon juice. Add the chicken cutlets, tossing well to coat evenly. Let marinate for at least 10 minutes.
03 - Preheat a grill or grill pan over medium-high heat. Grill the chicken cutlets for 4 to 5 minutes per side, or until fully cooked through with nice char marks.
04 - While the chicken cooks, mash the avocado with a pinch of salt and pepper in a small bowl until smooth.
05 - In a separate bowl, blend the mayonnaise and Dijon mustard until well combined.
06 - Lightly toast the sandwich rolls or ciabatta buns on the grill or in a toaster until golden.
07 - Spread the Dijon mayo on the bottom half of each roll. Layer with spinach or lettuce, a grilled chicken cutlet, cheese if using, tomato slices, red onion, and a generous spoonful of mashed avocado. Top with the other half of the roll.
08 - Serve immediately while warm and fresh.