Boil a pot of water to cook the pasta.

Slice the mushrooms and place them in a pan. You can start without oil because mushrooms have a lot of moisture. Dry sauté them to release the moisture. Once the mushrooms change color, mince the garlic and add it with a bit of olive oil.

Slice the onion and add it to the pan. Sauté until it turns translucent. Add the salmon. You don't need to cut the salmon as it will break apart when stirred with a spatula. Once the salmon is cooked, you can add the pasta to the boiling water. Remember to add salt to the water. Add the heavy cream and capers.

While the pasta is cooking, scoop a little pasta water into the cream sauce. Reduce the heat to low and season with half a teaspoon of salt. Then add the spinach. Cook the pasta until it's about 80% done, not fully cooked, and then add it to the white sauce and stir-fry briefly.