Lemony potato and green bean salad - chinese grandma

Ingredients

• 1 1/2 tablespoons dijon mustard
• 1/4 cup lemon juice
• 2 tablespoons extra-virgin olive oil
• 1 clove garlic, smashed
• 1/2 teaspoon salt
• 1/2 teaspoon pepper
• Cayenne pepper, to taste
• 1 1/2 pounds green beans, ends trimmed
• 1 1/2 pounds new potatoes, scrubbed well
• 1/4 cup finely chopped chives