Begin by rinsing the green beans under cool water and trimming their ends. Bring a pot of salted water to a rolling boil and add the beans, blanching them for 3-5 minutes until they are tender-crisp and bright green. As soon as they finish, immediately transfer the beans to an ice bath for 2-3 minutes to halt the cooking process. Drain and pat them dry with a clean towel.

In a medium bowl, whisk together freshly squeezed lemon juice, white wine vinegar, Dijon mustard, and optional sugar. Add a pinch of salt and pepper to taste. While whisking continuously, slowly drizzle in the vegetable oil until the mixture is fully emulsified and creamy.

In a large mixing bowl, combine the blanched green beans with the thinly sliced red onion. Pour the prepared lemon vinaigrette over the top, gently tossing the ingredients together to ensure every piece is well-coated with the dressing.

Finally, fold in the crumbled feta cheese and toasted walnuts for a delicious contrast of textures. Serve the salad immediately for the best crunch, or let it rest for 15-30 minutes at room temperature to allow the flavors to meld beautifully.