Trim the narrow ends off the cucumber. Cut 8 very thin rounds from the end of the cucumber if you plan to garnish with more cucumber slices.
Cut the cucumber vertically down the center. Using a mandarin or vegetable peeler cut 4 thin strips of cucumber the full length of the cucumber. If you are using a vegetable peeler lay the cucumber on the counter, flat side up and draw the vegetable peeler evenly from one end of the cucumber to the other.
Form a 2 ½ - 3" circle with the cucumber strip overlapping any excess of the strip. (If you assemble the salad on the plate you plant to serve it on it will work best. If not, have a wide spatula on hand that is broader than your cucumber circle so you don't lose ingredients when transferring the salad to the plate.)
Gently place about 3-4 Tbsps of the shredded lettuce as the base of the salad. Top with 1-2 Tbsps small sweet peas.
Clean and slice the mushroom caps as thinly as possible and cover the pea layer, overlapping the slices.
Mash the hard boiled eggs with a fork. Add the Boursin cheese and the mayonnaise and mix until smooth. Top the mushroom layer with ¼ of the egg salad.
Insert a bamboo skewer in each cucumber round to secure it.
Refrigerate 2-4 hours or up to overnight. When ready to serve, if moving the salad to a plate, ensure that you slide a spatula under each round that will completely cover the bottom so no ingredients spill out when transferring.
Pinch cucumber rounds in the center to make a flower shape and press into the egg salad topping. Garnish plate with 1 ½ grape tomatoes each, a teaspoon of mustard caviar or chopped pickle and a bit of fresh herb garnish.