Monkfish with shrimps (Rap amb gambes)

This is a recipe from my mum. And it tastes great.
You will need

monkfish
lemon
salt
oil
onions
tomatoes
shrimps

and mayonnaise

Clean the monkfish and put is with lemon and salt. Keep it in the fridge. Then you have to peel (or shell) the shrimps first and you have to cook the shells with oil that you will use for cooking the monkfish. Put the shrimps shelled in the fridge. The oil should be impregnated with the taste of this. Then you filter it and throw the shells away. Once you finished you have to put this oil in a big saucepan or casserole with 2 onions sliced. Once the onion is done you have to add the monkfish Let this cook for 5 minutes. Then add 5 spoons of fried tomatoe and the shrimps. Just 5 minutes more. Then you can let it like it or add homemade mayonnaise on the top while it is in the saucepan. And shake it well. You can put it in the oven to gratin a little bit the mayonnaise (be careful, it gratins very fast).