Slice and de-water the cucumbers. Thinly slice the cucumber (aim for nearly see-through). Lay slices on paper towels, sprinkle with the salt, and let sit for 10 minutes. Blot very well with more paper towels. If they taste aggressively salty, give them a quick blot with a barely damp paper towel, then blot dry again. You want crisp, not drippy.

Mix the herb spread. In a bowl, stir together the cream cheese, mayonnaise (if using), lemon zest, lemon juice, dill, chives, black pepper, and garlic (if using). Taste and adjust lemon, pepper, or herbs.

Prep the bread. Lay out all 12 slices of bread. If using butter, spread a very thin layer on one side of each slice. Then spread the herb cream cheese mixture on top of the butter (or directly on the bread), going all the way to the edges. This helps seal the bread and keeps the sandwiches neat.

Layer cucumbers. Arrange cucumber slices over the spread on 6 of the slices in slightly overlapping rows so every bite gets cucumber. Do not pile them too thick. Delicate is the goal.

Close, press, and chill briefly. Top with the remaining 6 slices, spread-side down. Press gently with your palms. For the cleanest cuts, stack the assembled sandwiches on a board, then wrap the whole stack tightly in plastic wrap (or parchment plus plastic wrap) and chill for 10 to 15 minutes.

Trim and cut. Use a serrated knife to trim off crusts, then cut each sandwich into 4 squares or 4 small triangles (cut diagonally once, then diagonally again). Wipe the knife between cuts if needed for sharp edges. This yields 24 tea-sandwich pieces.

Serve. Arrange on a platter, garnish with a few dill fronds if you want, and serve chilled or cool room temperature.