Start by slicing the boneless, skinless chicken breasts horizontally to create four thin cutlets. Pat them dry with paper towels.

In a bowl, rub the chicken cutlets with olive oil, paprika, salt, pepper, and optional chili flakes. Let the seasoned chicken rest for 10 minutes.

Preheat your grill to medium-high heat. Grill the chicken cutlets for 3-4 minutes on each side until cooked through.

Melt the unsalted butter in a small saucepan over low heat. Add minced garlic and chopped parsley, stirring for 2-3 minutes until fragrant.

Brush both sides of the thick-cut sourdough bread with the garlic butter and toast in a skillet for 2-3 minutes on each side until golden brown.

In a small bowl, mix the mayonnaise, Dijon mustard, and lemon juice until smooth. Adjust the tanginess as needed.

Spread a layer of lemon-Dijon mayo on each piece of toasted sourdough. Layer with lettuce, grilled chicken cutlets, and slices of tomatoes.

Gently press the sandwich together, cut in half if desired, and serve warm.