If your 10 oz chicken breast is thick, butterfly it or pound gently to an even thickness of about 1/2 inch to ensure even cooking. Pat dry with paper towels.

Make the marinade

In a bowl, combine 2 tbs olive oil, 1 tbs lemon juice, 1 tsp minced garlic, 1 tsp smoked paprika, 3 tsp kosher salt, 1 tsp freshly ground black pepper, and 1 tsp dried oregano. Whisk until combined.

Marinate

Place the chicken in a shallow dish or resealable bag and pour the marinade over it, making sure both sides are coated. Let sit for at least 15 minutes at room temperature or up to 2 hours in the refrigerator. If refrigerating, bring to room temperature for 15–20 minutes before grilling.

Preheat the grill

Preheat an outdoor grill or grill pan to medium-high (about 400–450°F / 200–230°C). Oil the grill grates or brush the pan with a little oil to prevent sticking.

Grill the chicken

Remove excess marinade and place the chicken on the hot grill. Grill for about 4–6 minutes per side, flipping once, until the internal temperature reaches 165°F (74°C) and the juices run clear. If using cheese, place a slice on each breast in the last minute of cooking and close the lid to melt.

Rest

Transfer the grilled chicken to a cutting board and let rest for 5 minutes to allow juices to redistribute. Slice the chicken crosswise if desired for easier sandwich assembly.

Sandwich Build

Toast the buns

While the chicken rests, spread softened butter on the cut sides of the rolls or buns. Toast on the grill or in a skillet cut-side down for 1–2 minutes until golden and slightly crisp.

Prepare the sauce

In a small bowl, mix 2 tbs mayonnaise with 1 tsp Dijon mustard. Adjust seasoning to taste (add a pinch of salt and pepper if desired).

Assemble the sandwich

Spread mayonnaise-Dijon on both cut sides of the toasted buns. On the bottom bun layer 1 leaf of lettuce, 2 slices of tomato, and 2 slices of red onion. Place the grilled chicken breast (whole or sliced) on top, then add the cheese slice (if not already melted) and the second lettuce leaf if desired. Top with the bun crown.

Serve

Serve the grilled chicken sandwich immediately while warm. Optionally serve with pickles, fries, or a side salad.