Trim any excess fat from the chicken breasts. If they're thick, place each breast between plastic wrap and pound gently to an even thickness (about 1/2 inch) so they cook evenly.

In a bowl, whisk together olive oil, lemon juice, minced garlic, paprika, onion powder, kosher salt, and black pepper. Add the chicken breasts and turn to coat thoroughly. Marinate for at least 15 minutes at room temperature or up to 2 hours refrigerated. If refrigerating, bring to room temperature 20 minutes before cooking.

Cook Chicken

Heat a large skillet or grill pan over medium-high heat. Add a thin film of oil or nonstick spray if needed. When hot, add the marinated chicken breasts. Cook undisturbed 4–5 minutes on the first side until golden and juices start to appear at the top edges.

Flip the breasts, reduce heat to medium, and cook another 4–5 minutes until an instant-read thermometer inserted into the thickest part reads 165°F (74°C). If using thicker breasts, cover pan and cook an extra 1–2 minutes as needed. During the last minute of cooking, place a slice of cheddar cheese on each breast to melt.

Remove chicken to a plate and tent loosely with foil to rest for 5 minutes. Resting ensures juices redistribute and the cheese finishes melting.

Prepare Buns & Sauce

Meanwhile, split the buns and lightly spread butter on the cut sides. Toast them butter-side down in a clean skillet over medium heat until golden, 1–2 minutes.

In a small bowl, mix mayonnaise and Dijon mustard. Taste and adjust seasoning to preference.

Assemble (Makes 2 Sandwiches)

Spread about 1 tablespoon of the mayonnaise-mustard sauce on the bottom half of each toasted bun. Place a lettuce leaf on each bottom bun, then top with a rested chicken breast with melted cheddar.

Add 2–3 tomato slices and 2 pickle slices per sandwich (or to taste). Finish with the top bun, press gently to compact, and serve immediately.