Slice the cucumbers 1/8 inch thick. English (seedless) cucumbers are the pick — regular cucumbers work if you scoop out the seeds, since the seeds water out the salad.
Salt first: toss the cucumber slices with 1/2 tsp kosher salt, rest 15 minutes in a colander, then rinse briefly and pat dry. The salad stays crisp for hours and the dressing stays bright.
Whisk the lime juice, fish sauce (or salt), sugar, and chile until the sugar dissolves. Taste: it should be tart first, savory second, barely sweet.
How the mint is cut: tear large leaves by hand or stack, roll, and slice into thin ribbons (chiffonade). Do not chop mint with a heavy hand — bruised mint turns dark and bitter. Add the mint at the last minute; it wilts and darkens in acid if it sits.
Toss the cucumbers (and shallot, if using) with the dressing just before serving. Fold in the mint last. Taste and adjust lime or salt. Serve cold.