Preheat oven to 200°C. Heat oil in a large frypan over medium-low heat. Add onion and a pinch of salt and cook, stirring occasionally, for 10-12 minutes until onion is very soft but not coloured. Remove from the pan and cool slightly.
Whisk eggs, cream, mustard and onion together in a bowl and season to taste. Divide mixture between 2 x 500ml- capacity round ovenproof dishes. Top with ham and brie slices and place in the oven. Bake for 20-25 minutes until the edges are golden and puffy and the centre is cooked. Serve with tomato chutney and cornichons.