Prepare fettuccine according to package directions, reserving 1 cup cooking water. Set aside.
Into the same pot used for cooking pasta, add butter. Set heat to medium and allow butter to melt.
Add spinach leaves and cook 2-3 minutes, until beginning to wilt.
Add garlic, salt, and pepper. Continue cooking another 1-2 minutes, until spinach is wilted.
Add cooked pasta, parmesan cheese, and 1/2 cup of the reserved pasta water. Gently toss to coat, adding more reserved water as needed.
Adjust seasoning to taste.
To serve:
Divide paste between serving bowls. Sprinkle with additional Parmesan cheese and red pepper flakes. Serve hot.