Classic fish gratin

This is not the first time I am trying this Swedish classic at home but somehow, the sauce was never right. I took this in a couple of restaurants in town and always enjoyed it so much that I had, at some point, to find THE recipe. I think this is it! It will need some small adjustments, but it's almost perfect!! The recipe should give you 4 good portions and is adapted from this recipe.

I made the dish in one of our nice "ultra", the baking dish we have from Tupperware that we can put in the oven/fridge (very nice to have!). Also, stuff does not really stick to it so its nice. If you do not have it at home, butter a baking dish (size roughly 1.5-2 L, or 20x27 cm).

Peel and cut roughly 700 g potatoes. Cook with your preferred method. In my case, I boiled them because there was just to much! In the recipe, they put quite a bit of butter and mash the potatoes, but instead I used 50g of butter (instead of the 200 g recommended in the original) and some of the delicious sauce (see next paragraph). The idea is that the sauce is already spiced and nicely salted so no need to add anything else. Add until the mashed potatoes are smooth and nice, something like 75 ml of sauce.

Melt 50 g of butter in a saucepan on medium heat. Whisk in 3 tbsp of flour. You need to make sure that you do not have any lumps.  Add 850 mL of milk, pouring very slowly for roughly the first third of the volume, making sure no lumps are forming. I always tend to put the heat too warm and I end up removing it form the heat, spending 5 minutes unmaking the lumps... Bref, be patient and add slowly the milk at first. It should eventually be more "watery" so then you can be a little bit faster, stirring it all the time. Add 150 mL of cream and a cube of fish stock. Stir well and let the sauce thicken. Add 3 tbsp of mustard (not "prepared mustard": try something like Dijon or honey-Dijon), salt and pepper to taste, a big pinch of nutmeg, a tablespoon of dill a tablespoon of chives. We have fresh/frozen herbs, this is perfect, but if you use dry herbs, begin with half and increase to taste. Taste the sauce and adjust to your liking.

As mentioned before, pour some of the sauce to mash the potatoes. Spoon the mashed potatoes around the baking dish. In restaurants, they use decorating tip to make a fancy shapes (like decorating a cake) so if you feel like it please do! I did not have such thing, so I simply spooned it all around the baking dish. The mashed potatoes need to be all around, making a sort of bowl of potatoes: the sides are thicker and the bottom is thinner. Cut 400 g of white fish (we had cod) in bite-size pieces. In the potato bowl, pour some of the sauce, lay out pieces of the fish, pour some more sauce... alternating until both are finished. Scatter 100 g of shrimps (of course without the shell) on top of the sauce and put in the oven at 180 C for 30 minutes, or until the potatoes are crispy and the sauce is bubbly and slightly brown.