Saute onions in oil and butter until caramelized and golden brown, then add thyme and a pinch of salt. Remove from pan and allow to cool. In a bowl, add heavy cream, eggs, nutmeg, and onions, season with salt and pepper. Pour mixture into tart shell and bake in a preheated 350 degree oven for 35 to 40 minutes or until center does not jiggle.
Courtesy Cheryl Smith