Beef tenderloin in a mustard crust

1. In a frying pan, brown the meat over high heat. Let drain and cool on a rack. Brush the meat with mustard and wrap it in a layer of cling film. Cool in the refrigerator for 10 minutes.
2. Preheat oven to 180°C (gas mark 6).
3. Mince the onions and fry them in a pan with oil. When the onions have already sweated well, add 3 tablespoons of sugar and a little water several times until you have a compote. Put the onions on a plate with paper towels to dry them as much as possible.
4. Take the meat out of the fridge, unwrap it from its cling film, place the onions around the roast with the help of the cling film so as to obtain a new sausage. Put the whole thing in the refrigerator for 5 minutes.
5. Prepare the puff pastry and an egg yolk in a container. Unwrap the papillote with the roast surrounded by onions and place it on the puff pastry. Wrap the puff pastry around the roast and brush with egg yolk.
6. Put it in the oven at 180°C for 30 minutes. Take the meat out of the oven and let it rest for 10 minutes so that it is tender and you can cut it without the pastry breaking.

Serve hot with a good mayonnaise and French fries.