Boil the eggs: Place eggs in a saucepan and cover with cold water by 1 inch. Bring to a gentle boil, cover, remove from heat, and let stand for 10 to 12 minutes.

Cool and peel: Drain, shake the pan to lightly crack shells, then cover eggs with ice water for 10 minutes. Peel under running water to make it easier.

Mash the eggs: Pat eggs dry and place in a bowl. Use a fork or pastry cutter to chop into small, fluffy pieces. Avoid over-mashing into a paste.

Mix the dressing: In a small bowl, stir together mayonnaise, Dijon, lemon juice, a pinch of salt, and pepper. Start with less mayo; you can add more if needed.

Combine: Fold the dressing into the eggs. Add celery, scallions or chives, and herbs if using. Taste and adjust salt, pepper, and lemon.

Prep the bread: Lay out slices and lightly butter one side of each. Butter creates a moisture barrier so the bread stays soft but not soggy.

Assemble: Spread a generous, even layer of egg salad on half the slices. If using watercress or thin cucumber, add a single, light layer. Top with remaining bread, buttered side in.

Trim and cut: Use a sharp serrated knife to trim crusts. Cut each sandwich into 2 or 3 rectangles or into neat triangles. Wipe the knife between cuts for clean edges.

Chill briefly (optional): For tidier shapes, cover and refrigerate for 20–30 minutes before serving. Serve cool, not cold.