01 - Butterfly the chicken breasts by slicing them horizontally through the center, creating 4 thinner, even cutlets for faster and more uniform grilling.
02 - In a mixing bowl, combine the olive oil, garlic powder, smoked paprika, salt, black pepper, and lemon juice. Add the chicken cutlets and toss thoroughly to coat all surfaces. 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 chicken cutlets on the grill and cook for 4 to 5 minutes per side until fully cooked through and nicely charred with grill marks. Internal temperature should reach 165°F.
04 - While the chicken cooks, halve and pit the avocado, then scoop the flesh into a small bowl. Mash with a fork and season with a pinch of salt and pepper to taste.
05 - In a separate small bowl, blend the mayonnaise and Dijon mustard together until smooth and well combined.
06 - Lightly toast the split sandwich rolls or ciabatta buns on the grill or in a toaster until golden and slightly crisp on the cut sides.
07 - Spread the Dijon mayo mixture 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 generous spoonful of mashed avocado. Cap with the top half of the roll.
08 - Serve the sandwiches immediately while warm and fresh for the best texture and flavor.