For shrimp, bring champagne, lemons and tarragon up to a simmer (I start with chilled champagne as the bottle will open without spilling).
Add shrimp and cook, just below a simmer until pink, about 3 minutes. Remove with a slotted spoon and cool.
Remove tail from shrimp and slice shrimp in half lengthwise. Toss shrimp with a splash of champagne vinegar and chill until ready to assemble.
To assemble, separate 16 endive spears and arrange on a platter. Spoon or pipe a teaspoonful of dip at the base of each spear.
Arrange 2 shrimp halves over dip (shrimp can curl over each other). Garnish with a petal from a flower (or a single flower if small).
For the Dip:
Stir all ingredients together and chill until ready to assemble.