In a large skillet, heat the oil and butter. When melted, add the onions and the thyme. Cook slowly, over medium-low heat, until the onions turn limp and brown.
Preheat oven to 350 degrees.
Using a mixer, beat the eggs. Add the dry ingredients and mix well over medium speed.
Lower the mixer speed and add the cream.
Grease a medium-sized gratin dish. Add the onions. Cover with the pudding mixture.
Bake 40 minutes or until browned and bubbly.