Preheat oven to 400 degrees. Place salmon, skin side down, on a baking dish. Season with sea salt and black pepper then squeeze lemon juice all over top. Roast salmon in oven for about 12 minutes or until it reaches an internal temperature of 145 degrees. Separate salmon filet(s) from skin and cut into bite-sized pieces.
Meanwhile, cook pasta per package directions in salted water, drain, and set aside.
While pasta and salmon cook, add 1-2 tablespoon olive oil to a large skillet over medium-high heat and add chopped onion. Stir and cook until fragrant and translucent (about 5 minutes). Add garlic, mushrooms, then butter and allow butter to melt, stirring constantly.
Add heavy cream, white wine, lemon zest, sea salt, black pepper, cayenne pepper, smoked paprika, and crushed red pepper to pan. Stir until uniform and bring to a boil (about 4 minutes).
Reduce heat to medium-low, stir in shredded Parmesan cheese until melted, and let sauce simmer for about 5 minutes, stirring every minute or so.
Turn off heat and stir in spinach until wilted. Add pasta and salmon pieces to pan and gently toss to coat in the sauce. Garnish with parsley and more shredded Parmesan, if desired.