1. In a large pot, five quarts or more, bring water to a boil. Add two tablespoons kosher salt and pasta. Boil for eight to 10 minutes until pasta is al dente (firm to the tooth), stirring to prevent sticking. Drain and toss with one tablespoon oil. Cover to keep warm.
2. Heat oil in a medium skillet until extremely hot. Sear the salmon cubes, turning them so that all sides are cooked. Cook until fish cubes are opaque except in the center (the center should still be slightly translucent); about three to four minutes. Remove from skillet and set aside.
3. In the same skillet, heat two tablespoons butter until melted. Sauté the onion over medium heat until softened. Add parsley and peas or spinach. Add kosher salt and pepper and sauté five minutes longer, or until moisture has evaporated.
4. Remove from heat. Add salmon cubes and vegetable mixture to the spaghetti.
5. Wipe out skillet with a paper towel and melt remaining butter. When melted, stir in flour and whisk to a smooth consistency. Turn off flame, then add heavy cream and stir well to combine. Add it to pasta and toss to coat the spaghetti, salmon, and vegetables. Adjust seasonings with salt and pepper to taste. Serve immediately.