1- In a large pot pour your water, then add your onion skins, thyme, bay leaf and your carcass or veal bone, salt and cook for 1/2 hour at low boil, skimming from time to time.
2- In another pot melt your butter, then "fall" your sliced onions in it, salt and pepper then let them sweat slowly covered for 20 minutes.
3- Then coat with the 40g of flour, then using a strainer moisten with your onion cooking liquid, and then cook thus for about ten minutes.
4- Turn on your oven in Grill position.
5- Using a skimmer distribute your onions in your containers then pour the juice over them and cover them with your croutons topped with your Gruyère and then bake in the oven until golden.
The quality of this recipe is that thanks to the cooking of the "skins" your gratinée will have an inimitable taste and thus you will be up to date by using the full qualities of this product.
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