Preheat the oven to 220°C/ gas 7. Heat 1 tbsp of the oil in a skillet and cook the onions, stirring over a low heat for about 5 minutes. They will be soft and golden. Combine the cream, salt, pepper and nutmeg. Stir the cream mixture into the onions. Heat the remaining oil in the skillet and fry the bacon until it is lightly browned. Drain on paper towels and reserve.
Oil a 14x9in baking sheet. Roll out the shortcrust pastry until it is slightly smaller than the baking sheet and place it on the sheet. Spread the onion mixture over the dough and dot with the bacon. Bake for 20 minutes or until the tart is lightly brown. Serve hot from the oven.