Melt the butter and oil together in a large skillet.
Add the onions, thyme and salt to the skillet. Cook on medium-low heat until the onions are completely translucent and caramelised - about 30 minutes. Remove from heat and let cool slightly.
While the onions are cooking, preheat oven to 200C / Gas mark 6.
Combine the eggs and the single cream in a bowl. Add the caramelised onion mixture and the black pepper.
Roll out the puff pastry in a buttered pie or glass dish. (Make sure the pastry has sufficient turned-up edge to hold the onion mixture.)
Add the mixture to the pastry and bake for 25 minutes or until the crust and top are slightly browned.
Allow to cool, then slice and serve with a mixed salad.
