Bring a large pot of salted water (1 to 2 tablespoons salt) to a rolling boil. Add the pasta and cook for 8-10 minutes, stirring occasionally, until al dente with a slight firmness when you bite into it. Before draining, scoop out 1 cup of the starchy pasta cooking water and set aside. Drain pasta in a colander and toss with a drizzle of olive oil to prevent clumping.

Pat salmon fillet completely dry with paper towels, then season both sides generously with salt, pepper, and a light sprinkle of lemon juice. Heat 2 tablespoons olive oil in a large skillet over medium heat until it shimmers. Place salmon flesh-side down and cook undisturbed for 4-5 minutes until a golden crust forms. Flip carefully and cook another 4-5 minutes until internal temperature reaches 145°F and fish flakes easily. Remove to a plate, let rest for 2 minutes, then flake into bite-sized chunks.

In the same skillet (don't wipe it out), add minced garlic and sauté for 30 seconds, stirring constantly, until fragrant but not browned. Immediately add chopped spinach and cook for 2-3 minutes, stirring frequently, until completely wilted and bright green.

Lower heat to medium-low and pour in heavy cream, using a wooden spoon to scrape up flavorful bits from pan bottom. Let warm through for about 1 minute. Gradually sprinkle in Parmesan cheese while stirring constantly in a figure-eight pattern until completely melted into a smooth, velvety sauce that lightly coats the back of your spoon.

Stir in lemon zest, fresh lemon juice, and red pepper flakes if using. Let sauce bubble gently for 2-3 minutes. Taste and adjust seasoning with more salt and pepper as needed.

Add drained pasta directly to skillet and use tongs to toss everything together until every strand is coated. Gently fold in flaked salmon, keeping some larger chunks intact for presentation. If sauce looks too thick, add reserved pasta water one tablespoon at a time, tossing between additions, until it flows but still clings nicely to noodles.

Transfer to warm serving bowls or plates immediately. Garnish with fresh chopped parsley and extra Parmesan if desired. Serve right away.