Butter one side of each bread slice evenly. Heat a nonstick skillet or griddle over medium heat. Place one slice butter-side-down in the pan, top with the cheese slice, then place the second bread slice butter-side-up on top. Cook 2–3 minutes until the bottom is golden; flip carefully and cook another 1–2 minutes until cheese is melted and both sides are golden. Remove and let rest 30 seconds, then cut in half if desired.

Classic dress (lettuce)

Wash the lettuce leaf, pat dry, and tear or trim to sandwich-appropriate size. In a small bowl, mix the mayonnaise with lemon juice, a pinch of salt and black pepper. Taste and adjust. Spread the dressed mayonnaise lightly over the inside of one bread slice (or directly on top of the melted cheese) and place the dressed lettuce leaf on the sandwich.

Tomato pickle

Dice the half tomato into very small pieces (or thinly slice if preferred). In a small bowl combine the tomato with vinegar, sugar, 1/4 tsp salt and red pepper flakes if using. Mix and let sit 3–5 minutes to macerate — this quick pickle brightens the tomato and reduces excess juice. Spoon the tomato pickle onto the dressed lettuce on the sandwich, gently pressing to keep it contained.

Onion

Thinly slice the quarter onion into rings or half-rings. If raw onion is strong, toss the slices with 1/2 tsp lemon juice and 1/8 tsp salt and let sit 2–3 minutes to mellow. Drain any excess liquid and arrange a few rings on top of the tomato pickle (or under the lettuce, to suit preference).

Finish

Close the sandwich, press gently, slice if desired, and serve immediately while warm.