Bring a large pot of salted water to a rolling boil. Add the fettuccine or linguine and cook until al dente, about 8-10 minutes. Reserve 1 cup of pasta water before draining.

While pasta is cooking, season the salmon fillet with salt, pepper, and lemon juice. In a skillet, heat 2 tablespoons of olive oil over medium heat. Add the salmon and sauté for 4-5 minutes per side, or until it flakes easily with a fork. Set aside.

In the same skillet, add the minced garlic and sauté for about 30 seconds until fragrant. Next, toss in the fresh spinach, cooking until wilted, approximately 2-3 minutes. Reduce the heat, pour in the heavy cream, and gradually stir in the Parmesan cheese until melted and well combined. Mix in the lemon zest, juice, and optional red pepper flakes, and let it simmer for 2-3 minutes.

Gently toss the cooked pasta into the creamy sauce, ensuring it's evenly coated. Fold in the flaked salmon, and if the sauce is too thick, adjust the consistency with reserved pasta water as needed.

Plate the delicious salmon spinach pasta immediately, garnishing each serving with freshly chopped parsley. Enjoy this delightful dish while it's fresh!