Bring a large pot of salted water to a boil and cook the pasta until al dente. I always reserve some pasta water before draining. This water is key for adjusting the sauce later.
In a large skillet, melt butter with olive oil over medium heat. Add minced garlic and cook gently until fragrant. Be careful not to brown it, as garlic turns bitter fast.
Add fresh spinach to the skillet and stir until wilted. This takes only a minute or two. The spinach should stay bright green and tender.
Add the drained pasta directly into the skillet. Toss everything together so the noodles get coated evenly. Pour in some reserved pasta water to loosen the sauce.
Season with salt, black pepper, and red pepper flakes. Turn off the heat and sprinkle in Parmesan cheese. Toss again until everything looks glossy and well mixed.