I like to slightly undercook my fettuccine because it finishes cooking in the sauce. Reserve a cup of pasta water before draining, as it helps adjust the sauce consistency later.
In a blender, combine milk, cottage cheese, sun dried tomatoes, parmesan, garlic, red pepper flakes, salt, and black pepper. Blend until smooth, scraping the sides as needed.
Warm the sauce over low heat in a skillet. Add half a cup of reserved pasta water to thin it out, ensuring it doesn’t simmer, or it may curdle.
Stir the cooked pasta into the sauce gently. Add more reserved pasta water, 2 tablespoons at a time, if the sauce is too thick.
Fold in torn spinach, toasted walnuts, and parsley. The heat from the pasta will wilt the spinach perfectly. Finish with extra parmesan and walnuts for garnish.