Boil a large pot of salted water and have a bowl of ice water ready. Add the 2 pounds green beans the pot and cook them for 2-3 minutes. Then immediately place them in the bowl of ice water.
In a large sauce pan or cast iron pan on the stove, heat the 3 tablespoons extra virgin olive oil over medium heat. Add the 3 cloves garlic and cook over low heat for 1 minute. Use a slotted spoon (or use a colander to drain) to add the blanched green beans to the pan and cook for 2-5 minutes until they reach your desired texture. I like mine to be more crisp than soft.
Zest the 1-2 lemons and stir into the pan. Season with salt and pepper. Stir in ¼ cup shredded parmesan if using, and cook over low heat until the cheese slightly melts. Serve hot or at room temperature.
Notes
What texture to cook the green beans depends on your preference. I personally like green beans that retain a bit of crispness - not soft/soggy - so I blanch them briefly and just cook them for a few more minutes on the stove. However, you can cook them longer if you like them softer.
Can I use frozen green beans? I haven't made this recipe using frozen green beans, and because it's such a light and fresh dish I prefer to make them with fresh beans for the best flavor. However, I do think you could use frozen green beans in a pinch. Don't blanch them first, just add them right to the pan.
Make ahead: You can blanch the green beans up to a day in advance. Place them in an airtight storage container and keep them in the fridge until ready to use.
How to store: Transfer any leftover green beans to an airtight container and store them in the fridge for up to 5 days. Reheat in a pan over medium heat or in the microwave.