Pre-heat the oven to 200°C. Cut the potatoes into small cubes and arrange on a baking tray covered with baking paper. Drizzle with olive oil and season with salt and pepper. Place in the oven for 30-35 minutes until golden brown.
Clean the green beans and cut them in half lengthwise. Add to a microwave-safe bowl with 30 ml of water. Cover with cling film and place in the microwave for 5 minutes at 800 W, or until cooked through.
Finely chop the spring onion and celery.
In a blender/food processor, add one of the spring onions, celery, anchovy filets (or alternative: dark soy sauce), baby spinach, mustard, garlic, sour cream, mayonnaise, and lemon juice. Mix/blend until you have a smooth green sauce. Add some water if the consistency is too thick. Season to taste with salt and pepper.
Add the potatoes to a serving bowl together with the cooked green beans and add the green sauce. Toss until everything is well coated. Serve with the other spring onion.
