In a small bowl, mix together the butter, parsley, chives, shallot and lemon zest.
Lay the bread slices out on a clean work surface. Spread 2 teaspoons of the compound butter over each slice of bread. Trim the cucumber slices so that they are the length of the bread slices. (Save the scraps for salads.) Lay 4 pieces of cucumber across 1 slice of buttered bread--it is okay if the cucumbers overlap. Repeat on 5 more slices of bread. Season the top of the cucumbers lightly with salt. Next, add a single layer of radishes over the cucumbers, about 12 rounds per slice of bread.
Toss the frisee with the lemon juice and a pinch of salt. Divide the frisee evenly over the radishes. Top the frisee with the remaining 6 slices buttered bread. Cut each sandwich into thirds with a serrated knife to make small rectangles. Serve immediately.