Bring a large pot of water to a boil and cook the pasta according to the package directions. While the pasta cooks, make the spinach mixture.
Reserve ¼ cup of pasta water, drain, and return the pasta to the pot.
Sauté:
In a small saucepan over medium heat, combine the olive oil, butter, garlic, salt, and red pepper flakes. Cook the mixture for about 2 minutes, stirring frequently, until the garlic is golden and fragrant, but not burned.
Reduce the heat to low, add the chopped spinach, and quickly sauté until just coated with olive oil, about 1 minute. Remove from heat. It doesn't take long to wilt the spinach.
Combine:
Pour the spinach mixture over the pasta, add the Parmesan cheese, and toss with kitchen tongs until all the pasta is coated. To thin out the spinach mixture, add a little of the reserved pasta water (making sure it's still hot) while tossing to coat.