1. Prepare an ice-water bath; set aside. Coarsely chop enough tarragon leaves to make 6 tablespoons, and set chopped leaves aside. Reserve remaining sprigs for garnish. Bring a small saucepan of water to a boil; add 1 teaspoon salt. Add scallions, and cook until bright green and just tender, about 1 1/2 minutes. Add tarragon leaves, and cook until bright green, about 20 seconds; immediately transfer scallions and tarragon to the ice-water bath. Let cool completely.

2. Drain scallions and tarragon, then squeeze dry in a clean kitchen towel. Coarsely chop, then transfer to a food processor; pulse until finely chopped. Add mustard. With machine running, add oil in a slow, steady stream; process until smooth and thickened, about 1 minute. Transfer to a small bowl; stir in creme fraiche. Cover, and refrigerate until ready to serve (up to 2 days).

3. Prepare an ice-water bath; set aside. Place a dozen eggs in a saucepan large enough to hold them in a single layer; cover with cold water by 2 inches. Bring to a boil, then reduce heat, and simmer 1 1/2 minutes. Using a slotted spoon, transfer eggs to the ice-water bath, and let cool. Meanwhile, repeat with remaining eggs. Peel eggs, dipping them in cold water if needed to remove shell pieces. (Peeled eggs can be refrigerated in an airtight container overnight.)

4. Pick 4 dozen small leaf clusters from remaining tarragon sprigs; set aside. Halve eggs lengthwise. Arrange blini on a platter. Spoon 1/2 teaspoon sauce in center of blini. Top each with half an egg, 1/4 teaspoon roe, and a tarragon cluster.