Heat 2 tablespoon olive oil in a large skillet over medium-low. Add sliced garlic and cook 3 minutes until soft and fragrant but not browned. Add red pepper flakes; cook 30 seconds.
Add vegetable broth and bring to a boil. Add penne rigate and stir. Season with salt, pepper, and nutmeg.
Reduce to a vigorous simmer. Cover and cook 14–16 minutes, stirring every 4 minutes, until penne is al dente and about ½ cup liquid remains.
Remove from heat. Drizzle in remaining 1 tablespoon olive oil and stir 30 seconds to emulsify the sauce. Stir in Parmesan.
Pile spinach over the pasta, cover the pan, and let sit 60–90 seconds until wilted. Add lemon zest and toss gently.
Divide into bowls. Dollop cold ricotta (about 3 tbsp) over each serving. Serve immediately without stirring in the ricotta.
