Spinach soup

Name: Spinach soup
Prep time: 10 min
Cook time: 25 min
Total time: 35 min
Cuisine: British
Ingredients: 25g butter; 1 bunch spring onions chopped; 1 leek (about 120g), sliced; 2 small sticks celery (about 85g), sliced; 1 small potato (about 200g) , peeled and diced; ½ tsp ground black pepper; 1l stock (made with two chicken or vegetable stock cubes); 2 x 200-235g bags spinach; 150g half-fat crème fraîche