Cook the pasta in salted water, according to the package instructions, until al dente. Remove from water and set aside. Save 1 cup of the pasta water for making the sauce. This can be done while cooking the onion and spinach, or ahead of time.
In a large skillet over medium heat, melt the butter. Add the onion and cook until soft and translucent. Add the garlic and red pepper flakes. Cook 1-2 more minutes until the garlic is fragrant.
Add the fresh spinach, cook while stirring gently, until the spinach is mostly wilted.
Move the spinach and onion mixture to one side of the pan and add the sour cream. Stir on that side of the pan to warm the sour cream and get it smooth. Slowly add the milk to the sour cream and stir to mix.
Slowly incorporate the spinach and onion mixture until the sauce comes together. Taste and add the salt and pepper to taste.
Add the pasta to the pan and stir to coat the noodles with the sauce. If the sauce is clumping or separating, slowly add the pasta water and stir to combine.
Remove the pan from the heat and serve warm. Garnish with fresh chopped parsley, grated parmesan, more red pepper flakes