Cook the spinach according to the package directions; drain well, pressing out excess liquid. In a medium saucepan cook onion in the 3 tablespoons butter or margarine until tender. Stir in the flour, tarragon, and pepper. Add milk all at once. Cook and stir until mixture is thickened and bubbly. Stir about half of the hot mixture into eggs; return all to saucepan. Stir in cheeses and spinach. Set aside.
Unroll the phyllo dough; remove 1 sheet of phyllo dough at a time. (As you work, cover the remaining phyllo dough with plastic wrap to prevent it from drying out.) Lightly brush the first sheet with some of the 1/2 cup butter or margarine. Fold phyllo in thirds lengthwise; brush the top with butter.
Place one end of the folded sheet in the center of a 12- or 14-inch pizza pan, extending it over side of pan. Repeat with the remaining phyllo dough and butter; arrange strips spoke-fashion evenly around pan. (The ends of each sheet will overlap in the center and be about 3 inches apart at outer ends.)
Spread the spinach mixture in an 8-inch circle in the center of the pastry. Starting with the last phyllo strip placed in the pan, lift the end of the leaf up and bring it in toward the center of filling. Holding end with both hands, twist end several times; coil and tuck end under to form a rosette. Lay rosette over filling, leaving a 3-inch circle in center (filling should be visible).
Repeat with the remaining phyllo strips in the reverse order in which they were placed in the pan. Drizzle any remaining butter or margarine over all. Bake in 375 degree F oven for 30 to 35 minutes or until golden. Serve warm. Cut into wedges to serve. Makes 10 appetizer servings.