In a bowl, add the salmon cubes and cover with the juice from one lemon.
In a deep sauté pan, melt the butter with olive oil over medium-low heat. Once melted, add the onions and sauté for 6-8 minutes or until translucent.
Add the salmon cubes to the pan, season lightly with salt, and cook for a minute or two, turning them often.
Increase the heat, pour in the juice from the second lemon, and allow it to evaporate. Remove the pan from heat.
In a large pot, bring water to a boil for the pasta. Add salt once it starts to boil and bring to the boil again. Cook the pasta al dente according to the instructions on the packet. Before draining, reserve a cup of pasta water.
In a blender, combine half the cooked salmon with half a cup of the saved pasta water. Blend until smooth, adjusting the consistency with more pasta water as necessary.
Pour the blended salmon sauce back into the sauté pan with the remaining salmon cubes.
Add the drained pasta to the pan and cook with the salmon mixture over medium heat for an additional 1-2 minutes, stirring frequently. If the mixture becomes too dry, add more of the reserved pasta water.
Once the pasta sauce reaches a creamy consistency, turn off the heat. Season with freshly ground pepper and garnish with lemon zest. Serve immediately and enjoy!