First 4 steps are done in advance. Cut your shrimp into small bite sized pieces. Set aside to chill.
Mix together your mayonnaise and dijon. Set aside, to chill.
Combine your onions, celery and carrots. Set aside, to chill.
Mix together your whole leaves of tarragon and soft light green celery leaves, from the heart. Could also use dill, fennel fronds, mache, chervil or parsley leaves. Cover with a moist towel and set aside, to chill.
When the time comes to serve, cut the bottoms off your endive. Cut about 1/2 inch from the base of the endive. Look for 24 nice big spears. With some of the unused smaller spears, you can slice them thin and add them to the shrimp salad (not too much, though ... endive can be bitter).
Take a small portion of the dijon mayo and spread it along the bottom of a plate or platter, to serve as a "glue" for the endive. This will help hold them in place, or else they like to roll around.
In a mixing bowl, blend the shrimp, the remainder of the dijon mayo and the veggies. Season with a little salt and pepper. Mix well.
Add a nice spoonful of salad to each endive spear.
Please each spear on the plate or platter.
Top each spear with a small amount of salad.
Serve!