Saute onions with oil and butter in skillet over low heat until well done, stirring often. Do not brown. Sprinkle with flour and continue cooking slowly about 2 minutes. Cool well. Beat eggs; add half and half, yogurt, salt, pepper, and nutmeg. Add to cooled onions. Sprinkle half the cheese on the bottom of the pre-baked pastry shell and fill with egg-onion mixture. Arrange pepperoni slices on top. Sprinkle with remaining cheese. Bake 10 minutes at 450 degrees; reduce heat to 300 degrees and bake 20 minutes more.
Source: Rock Springs Sweet Onion Festival Cook Book 1992