I make my Lemon Meringue filling in the microwave
Mix sugar,cornstarch and water together in a qt. glass casserole , cook for 1 1/2 minutes, stir, cook one minute longer or until thick stirring ever 30 seconds
beat a little of the hot mixture into the egg yokes ,and then add the rest of the egg yokes. cook 30 seconds
blend in the butter and lemon juice and rind, cool
Pour into baked pie shell and top with meringue
and bake in oven until brown.

Mine came out great, no lumps and easy to do.

Good luck.