Spaghetti with collard green and lemon

• 2 tablespoons extra-virgin olive oil 
• 2 garlic cloves, sliced 
• 1/4 teaspoon red-pepper flakes 
• 1 bunch collard greens (12 ounces), ribs removed, thinly sliced 
• 1/4 cup pine nuts, toasted 
• Grated zest of 1 lemon, plus more for serving 
• 2 tablespoons fresh lemon juice 
• Coarse salt 
• 12 ounces whole-grain spaghetti, such as farro 
• 1/4 cup finely grated Pecorino Romano, for serving

Heat oil in a large skillet over medium heat. Add garlic and red-pepper flakes; cook until tender, about 1 minute. Add collard greens and cook, stirring, until tender, about 5 minutes. Remove from heat; stir in pine nuts and lemon zest and juice. Season with salt. Meanwhile, cook spaghetti in a pot of boiling salted water until al dente, according to package instructions. Reserve 1 cup pasta water; drain pasta. Add pasta and reserved water to skillet, tossing to coat. Serve immediately, garnished with lemon zest and sprinkled with cheese.