Measure milk in a microwave safe measuring cup.
Heat in microwave until scalded.
Mix flour and sugar together in a heavy bottom pan.
Stir in hot milk with a whisk and cook over medium high heat stirring without stopping until the mixture is thick and just starts to bubble.
Remove from heat and add butter and vanilla, whisking until smooth.
Pour into a prebaked pie shell and sprinkle nutmeg over the top (fresh grated is best).
Cook at 375 degrees for 20 minutes. Pie should still be a bit wobbly in the center when its finished.
Cool completely before cutting.