Bring a pot of water to the boil (at least 3 liters).
Preheat oven to 350 F (180 C).
In the meantime, peel and very finely chop onion.
Clean mushrooms with a soft brush or a paper towel (do not wash with water!), cut off and discard very hard, bulky part of the stem (not all of it) and chop finely.
Wash, dry and very very finely chop dill leaves only, no stems until you have about 1 Tbsp of chopped dill.
Place salmon skin side down on a lightly greased baking sheet, salt and pepper generously and drizzle about 1 tsp of olive oil on each filet.
Put in the oven for 16-18 minutes depending on thickness. (set a timer for 16 minutes and poke into it to see if it's already done. Do not overcook or it'll become dry, cooke a minute at a time more until juuust done).
Heat a small pan over medium heat.
Add pasta to boiling water and cook as instructed on package (regularly 10-13 minutes are necessary, try one around minute 10 and keep a close eye to make sure you don't overcook it, they should be al dente). Set a timer for 10 minutes.
Once small pan is hot, add a dollop of olive oil and then chopped onions. Sauté onions until translucent, then add chopped mushrooms and sauté until soft.
Add creme fraîche and 1/2 tsp salt plus a couple turns to the pepper mill of pepper to the onions and mushrooms and stir until creme fraîche is completely melted. Remove from heat and cover to keep warm. Set aside.
Once pasta is al dente, drain and let cold water run over them while moving the colander around, this is to avoid them from sticking to each other, only do this shortly or the pasta will be cold.
Add pasta to a bowl, place salmon (which should be ready by now) on top and top with amazing mushroom sauce.