Take the yogurt and feta out of the fridge so they can come up to room temperature while the pasta water boils.
Bring a large pot of salted water to boil. Add pasta and cook according to the package directions until al dente. Reserve 1 cup pasta water. Drain and set pasta aside.
Add olive oil to the pasta pot and set it over medium-high heat. Add the garlic and red-pepper flakes and cook until the edges are just golden brown, 1 to 2 minutes. Lower the heat and add the drained pasta. Add spinach, tossing to combine and warm it through.
Gently stir in the yogurt, crumbled feta, herbs (if using), lemon zest and juice of ½ lemon. Add reserved pasta water as needed to loosen the sauce. Taste and add more salt, red-pepper flakes, and lemon juice as needed. Drizzle with olive oil and serve.
Total Time: 20 minutes
Prep Time: 10 minutes
Cook Time: 10 minutes
