Cook pasta. Cook the pasta in a pot of salted boiling water until al dente according to the package instructions. Reserve ¼ cup of pasta water before draining.
Prepare the salmon. Heat a large skillet over medium heat with 1 tablespoon olive oil and season each fillet with seasonings of choice. Cook the salmon for 3-5 minutes on each side or until cooked through to your liking (or depending on the thickness of your fillets). Remove from the pan and flake with a fork.
Saute the aromatics. Heat the remaining tablespoon of olive oil in a skillet over medium heat and saute the onion until soft and lightly browned. This could take anywhere from 5 to 8 minutes. Add in the garlic and stir until fragrant, about 1 more minute. Remove from the heat.
Make sauce. In a high-speed blender, combine the cashews, water, lemon juice, vinegar, salt, and pepper. Add the sauteed onions and garlic, then blend until completely smooth. Season to taste with more salt and pepper as needed. Thin with some of the reserved pasta water if needed.
Combine. Toss cooked pasta with sauce, capers, and flaked salmon. Garnish with parsley and serve.