Cook your pasta al dente in salted water (about 4 minutes from the time it starts to boil). Pour into a strainer and set aside. If you need to cook the salmon you can do this as the pasta is bubbling away (see *Recipe Notes below for instructions).
In the meantime toast the panko. In a non-stick pan melt 1 tablespoon of the butter, add the panko breadcrumbs and toast for a few minutes over a medium heat until golden stirring all the time. Remove from the pan and set aside. Preheat the oven to 425 F/ 220 C/ gas mark 7.
To prepare the sauce in a large non-stick pan melt 1 tablespoon of the butter, add the leek and cook gently for about 5 minutes until softened stirring often. Do not let the leek brown.
Add the remaining butter (½ tablespoon) and flour and stir until the flour has been thoroughly combined with the other ingredients (no dry bits visible). Pour in some of the milk and stir until the mixture becomes smooth. Then gradually add the rest of the milk, stock cube and mustard and cook until the mixture starts bubbling up stirring all the time. Remove from the heat, stir in the fromage frais and pepper to taste.
Assembling the recipe
Spread the pasta over the bottom of your casserole dish and place chunks of salmon on top. Pour the sauce over the whole thing and stir a little to make sure the sauce is evenly distributed.
Scatter the panko over the top and bake in the upper part of the oven for 15 minutes. Remove from the oven and serve immediately.
Notes
*You can cook salmon from raw using 2 methods: oven or stove top. To cook the salmon in the oven season and drizzle it with a little melted butter and bake in a preheated oven for 15 minutes at 375 F/190 C/ gas mark 5. The salmon does not need to be fully cooked - it will finish cooking with the pasta. Alternatively shallow fry the salmon in a non-stick pan in 1-2 tablespoons of oil/butter for 2 minutes on each side. Add into the casserole straight from the pan/oven.
Prepare ahead: To save time you can cook the pasta, salmon and the sauce ahead and once cooled refrigerate, covered, overnight. Microwave these ingredients briefly just to heat through before assembling the recipe (you may have to add a drop of water to the sauce to loosen its consistency). You can also toast the panko ahead and keep covered until you are ready to assemble the dish.
I do not recommend assembling the entire dish ahead as the pasta will have absorbed the sauce before it's had time to cook in the oven. The result will be soggy pasta and not enough creamy sauce.
You can use light sour cream or creme fraiche in the sauce instead of the fromage frais.
I do not recommend using fine breadcrumbs in this salmon pasta bake. Only panko breadcrumbs will guarantee a light and crispy topping. You can use more breadcrumbs if you like to create a thicker layer.
You shouldn't need to add any salt into the casserole (the stock cube is salty) but I recommend adding a good amount of white pepper.
Best served immediately. The pasta will absorb some of the sauce after a while.
Refrigerate leftover creamy salmon pasta bake for up to 3 days. Reheat in the microwave (add a drop of water if the consistency is too thick).