Heat pan to medium high. Prepare salmon by scaling and deboning it if needed. Cut slits halfway through the salmon filet and season with salt, pepper, and olive oil. If you have fresh rosemary, press that into the slits. Once pan is hot, add salmon and cook for 3 minutes on each side. Splash with white wine and lemon juice if it starts to burn.
Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Saute onion in a bit of butter in a large skillet over medium heat til onion becomes translucent. Add garlic powder, peas, and mushrooms, and cook over low heat for 4 minutes. Set aside.
Melt butter in a large skillet over medium low heat. Add in flour while whisking constantly. When it starts to darken from a yellow to a rich deep gold, slowly stir in milk or cream. Heat to just below boiling point, and then gradually stir in cheese until the sauce is smooth.
Toss in salmon and vegetables and cook for 1 more minute so everything is a consistent temperature. Serve over pasta.
