Bring a pot of well salted water to boil.
Heat oil over medium heat in a large, heavy nonstick skillet. Add garlic and cook until fragrant, about a minute; then stir in the greens and sauté until wilted.
Add pasta to boiling water and cook until al dente.
While the pasta cooks, turn the heat under the greens to as low as it goes and add the ricotta, lemon zest and a pinch of red pepper flakes, keeping everything warm while the pasta is cooking.
Reserve some cooking water and drain pasta; then add pasta to pan along with ¼ cup of reserved water. Toss well to coat. If you feel it is too dry, add up to 2 more tablespoons of cooking water. Season with salt and add more lemon juice, if desired.