Cook spinach according to directions; drain well, pressing out excess liquid. In saucepan, cook onion and garlic in 3 tablespoons butter or margarine until tender. Stir in flour, tarragon and pepper. Add milk all at once.
Cook and stir until mixture is thickened. Stir half of mixture into eggs; return egg mixture into remaining sauce. Add cheeses and spinach; set aside.
Unroll phyllo dough; cover with dampened towel. Remove 1 sheet of dough; brush lightly with some of the 1/2 cup melted butter. Fold the sheet in thirds lengthwise; brush top with butter. Place one end of the folded sheet in center of a 12- or 14-inch pizza pan, extending it over the side of the pan.
Repeat buttering of remaining phyllo dough, arranging strips spoke fashion evenly around pan. (The ends of sheets will overlap in center and be approximately 3 inches apart at outer ends.) Spread spinach mixture in an 8-inch circle in center of pastry. Starting with the last sheet of dough placed in pan, lift end of leaf up and bring toward center of filling.
Holding end with both hands, twist ends several times; coil and tuck end under to from a rosette. Lay rosette over filling, leaving a 3-inch circle in center (center should be visable). Repeat with remaining in reverse order they were placed. Drizzle remaining butter over all.
Bake in 375 degree oven 35 to 40 minutes or until golden. Serve warm or cool. Cut into wedges to serve. Makes 10 to 12 servings.
Preparation time: 20 minutes Baking time: 35 to 40 minutes