Start by bringing a pot of salted water to the boil to cook the pasta. Cook the pasta until about 2 minutes away from being properly cooked. So, not al dente yet. Drain, reserving 2 cups of pasta water.
Heat a non stick pan with olive oil. Once hot, place the salmon (I don't remove the skin) skin side up flesh side down into the pan. Don't overcrown the pan if the salmon is too big or the pan small, cook the salmon one a time.
Cook the salmon, medium to high heat, skin side down for about 3 minutes and gently flip to cook the skin side down for 4 minutes. While cooking, it you need to add olive oil, then do so, a tablespoon at a time. Use a large spoon to scoop the oil up and baste the salmon as it cooks, do this 2 to 3 times per side. Remove the salmon and rest on a plate.
Same pan, on medium to high heat, add the butter to melt and cook the onions until soft. Add the garlic cook for 2 to 3 minutes and while it cooks, add the fresh lemon juice. Continue cooking the onion and garlic for a total of 4 to 5 minutes. Add the capers and mix them in and then add the white wine, make sure your heat is not too low, if it is increase it slightly, you want to see the gently bubbles as it cooks.
Add the remaining tablespoon of butter and once the butter melts add the cooked spaghetti. Use a pair of tongs to lift the pasta as you mix it into the sauce. Regulate the heat by moving the pan on and off the stove just for a seconds as you mix. Pour in the reserved pasta water a dash at a time until you have the sauce you want.
Serve with the salmon, more melted butter (optional) and chopped fresh parsely. Taste before seasoning if you used capers.