Preheat the oven to 200c.
Prepare the pastry as per the packet instructions. Roll out and use to line a 12inch flan dish. Line with grease proof paper and baking beans and chill in the fridge for 15mins.
Bake the flan for around 25mins until lightly golden. Remove from the oven and allow to cool slightly before removing the paper and baking beans.
Reduce the oven to 180℃.
In a saucepan, add the butter and melt over a medium heat. Add the bacon lardens and cook for around 5-8mins.
Add the onion and cook for a few more mins until they being to soften. Add the thyme and stir for another minute. Remove from the heat.
In a large jug add the crème fraiche, cream, mustard and a large pinch of salt and pepper and stir together.
Spread the bacon mixture over the flan case, sprinkle with cheese and then gently pour the egg mixture over the top and scatter the tomato slices on top.
Bake on a high oven shelf for around 30mins until golden and just setting.
Leave to cool for a while and then serve hot or cold.