Warm Russian salad - Good Food Middle East

Ingredients

• 750g waxy potatoes (eg Charlotte, Maris Bard), peeled
• 250g carrots
• 2 large celery sticks, trimmed
• 175g fine French beans, topped and tailed
• 175g shelled peas or broad beans (about 600g/1lb 5oz in their pods)
• 2 medium gherkins, roughly chopped
• 1 tbsp salted capers, rinsed and roughly chopped (optional)
• 2 large egg yolks
• 1 tbsp cider vinegar or white wine vinegar
• 1 tbsp lemon juice
• 175g unsalted butter