Start by trimming the ends off the fresh green beans. Bring a large pot of salted water to a rolling boil, then add the green beans. Steam for 3-5 minutes until bright green and crisp-tender. Transfer to an ice bath immediately.

In a medium bowl, combine lemon juice with olive oil. Season with salt and pepper to taste, and whisk until emulsified.

Once the green beans have cooled, drain them well. Place them in a mixing bowl and add crumbled feta and chopped walnuts. Drizzle the vinaigrette over everything and gently toss to combine.

Cover the bowl with plastic wrap and chill in the refrigerator for 15 minutes. Serve at room temperature.