Trim the ends of the green beans and cut in half. Prepare an ice water bath.
Bring a large pot of water to a boil. Stir in baking soda and green beans. Cook green beans 4-5 minutes or until tender-crisp. Drain and place in the ice bath.
In a large bowl, whisk together olive oil, lemon juice, and Dijon mustard. Season with salt and pepper to taste.
Drain the beans from the ice bath. Toss the green beans and diced onion into the dressing. Allow to marinate for at least 30 minutes.
Top with feta and toasted walnuts just before serving.
Notes
*adding baking soda to the boiling water preserves chlorophyll, keeping the beans bright green.