Bring bread slices to room temperature if chilled. On one slice, spread 1 tablespoon mayonnaise evenly. On the other slice, spread 1 teaspoon Dijon mustard.

Assemble sandwich

Layer the sliced chicken breast evenly over the mustard-spread slice. Season the chicken lightly with 1/4 teaspoon salt and 1/4 teaspoon freshly ground black pepper. Place the cheddar slices on top of the chicken, then arrange the tomato slices and butter lettuce leaves.

Finish and toast (optional)

If you prefer the sandwich warm and slightly crisp, heat a nonstick skillet over medium heat and add 1 teaspoon unsalted butter. When butter foams, place the sandwich (mayonnaise side up) in the skillet, press gently with a spatula, and cook 2–3 minutes until the bottom is golden. Carefully flip and cook another 1–2 minutes until cheese softens and bread is toasted. If you do not toast, simply close the sandwich and proceed.

Season and serve

After toasting or closing the sandwich, slice in half. Taste and add the remaining 3/4 teaspoon salt and 1/4 teaspoon pepper over tomato slices only if needed (use sparingly). Add any optional add-ons (avocado, pickles, hot sauce) inside the sandwich before serving.

Simple Side Salad

Combine the mixed salad greens, cucumber slices, and thinly sliced red onion in a medium bowl.

In a small jar or bowl whisk together 1 tablespoon extra-virgin olive oil, 1 teaspoon fresh lemon juice, 1/2 teaspoon honey, 1/4 teaspoon salt, and 1/4 teaspoon freshly ground black pepper until emulsified.

Pour the dressing over the salad ingredients and toss gently to coat. Taste and adjust seasoning with more lemon, salt, or honey as preferred.

Optional Add-ons

If using avocado, slice and season lightly with a pinch of salt and a few grinds of pepper; tuck inside the sandwich before closing. Add pickles or a drizzle of hot sauce as desired.