Preheat a grill pan or outdoor grill to medium-high heat.

Brush chicken breasts with olive oil and season with salt and pepper.

Grill for 5–6 minutes per side, or until the internal temperature reaches 165°F (75°C).

Let rest for a few minutes, then slice thinly across the grain.

In a small bowl, combine the softened butter with minced garlic, Italian herbs, and red pepper flakes. Mix until evenly blended.

Spread garlic butter on one side of each bread slice.

Toast, buttered side down, on a skillet or griddle over medium heat for 2–3 minutes until golden and crisp.

Wash and dry the lettuce leaves. Slice the tomatoes and season lightly with salt and pepper.

Spread a thin layer of mayonnaise on the inside of each toasted bread slice.

Layer with lettuce, tomato slices, grilled chicken, and optional cheese or avocado.

Top with the second slice of bread, garlic butter side facing out.

Cut sandwiches in half and serve warm while the bread is crispy and the chicken juicy.