In a large saucepan, combine the 1/2 cup sugar, the flour, and cornstarch. Stir in milk. Add the 2 tablespoons butter. Cook and stir over medium heat until mixture is bubbly. Cook and stir for 2 minutes more.
Gradually stir 2 cups of the hot mixture into eggs. Return all the mixture to saucepan. Cook and stir over medium-low heat until mixture just begins to bubble. Remove from heat. Stir in vanilla.
Brush a 13x9x2-inch baking dish with some of the melted butter or margarine. Place 10 halved phyllo sheets in baking dish, brushing each with melted butter and keeping remaining phyllo covered with a damp (not wet) towel.
Pour custard over layers. Brush remaining phyllo sheets with butter and layer over custard.
Bake in a 350 degree F oven for 45 minutes. If phyllo gets too brown, cover with foil.
For syrup, combine the 1-1/2 cups sugar, the water, and lemon; stir until sugar dissolves. Bring to boiling. Reduce heat; simmer, uncovered, 15 minutes. Remove from heat. Remove lemon.
Cool dessert until custard is set (about 30 minutes). Pour syrup over dessert. Cover and chill. Cut into squares to serve. Makes 24 servings.