Combine flour, sugar, salt, eggs, milk, water and 2 teaspoons melted butter in a blender or processor for 1 minute, until smooth.
Heat remaining melted butter in 8-inch skillet (for 6-inch blintz) over medium heat just until bubbly. For each, pour about 2 tablespoons batter into hot skillet, rotating until batter covers the bottom. Cook until blintz bottom is golden brown, 1 to 2 minutes. Top should look cooked through and not wet. Turn out onto waxed paper; stack with sheet of waxed paper between each. When done, you should have 12.
Heat oil in large skillet over medium heat. Add onion; saute 5 to 8 minutes or until softened. Add beef; saute, breaking up with wooden spoon, 3 to 5 minutes or until no longer pink. Stir in flour, salt, pepper and paprika; cook, stirring, 1 to 2 minutes. Cool slightly before filling blintzes.
Heat oven to 400°F. Lightly grease 15 x 10 x 1-inch baking pan.
Spoon 2 tablespoons filling onto center of browned side of each blintz. Fold top and bottom of blintz over top of filling; fold in sides. Place blintzes, seam side down, on prepared pan.
Bake in 400°F oven 8 to 10 minutes or until lightly browned. Turn over. Bake another 8 minutes or until lightly browned. Serve immediately with sour cream if desired.