Heat oven to 450 degrees. Warm 2 1/2 cups milk, but do not scald. Separate 1 egg and beat white stiffly. Beat in 1/3 to 1/2 cup sugar, 1 teaspoon vanilla, 1/4 teaspoon salt, and 1/4 teaspoon nutmeg (optional). Combine egg yolk with 3 eggs. Stir gently and pour over beaten egg white. Beat slightly, only enough to blend. Add warm milk, stirring until sugar seems dissolved. Fill pastry shell, pour against a spoon. Place in oven, turn down heat to 425 degrees and bake for 30 minutes. The center will still be a bit shaky, but will firm on standing.