Egg gratin with mashed potatoes

INGREDIENTS
* 500 grams of potatoes
* 100 grams of butter
* 125ml cream
* 2 leeks
* 50 grams of flour
* 100ml of water
* 500ml of milk
* Salt
* 100 grams of cheese
* Pepper
* 4 eggs
* A teaspoon of vegetable oil
INSTRUCTIONS
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