Bring a pot of water to a boil for the pasta, but don’t use too much water (you want the pasta water to be starchy). For 8 ounces of pasta, I recommend 6 ½ to 7 cups of water (1.5-1.7 L). once the water is nearly boiling, add a generous amount of salt (2 to 3 teaspoons kosher salt).
Add the pasta and set a time for 2 minutes before the al dente cooking time listed on the package. Once the timer goes off, ladle out about ½ cup of pasta water and add the green beans to the pot. Cook for the remaining 2 minutes, until the pasta is al dente and the beans crisp-tender. Drain the pasta and beans.
Meanwhile, heat the olive oil in a 12-inch sauté pan over medium heat. Once warmed, add the green onions and garlic. Cook for 3 to 5 minutes, until the garlic is just golden, stirring frequently to prevent burning. Add the red pepper flakes and ¾ teaspoon kosher salt and cook for 30 seconds.
Add 1/4 cup of the pasta water to the pan and whisk to combine. Add in the hot cooked pasta and green beans, lemon zest, and 3 tablespoons lemon juice, tossing to combine with tongs. Add a bit more pasta water, as needed to bring the sauce together. Taste, adding more of the lemon juice as needed.
Finish with the chopped walnuts and season with salt and pepper to taste, and add more lemon zest as desired.