Leek and celeriac soup

Name: Leek and celeriac soup
Prep time: 5 min
Cook time: 30 min
Total time: 35 min
Cuisine: European, British
Ingredients: 2 medium leeks (rinsed, trimmed and shredded); 2 medium celery sticks (rinsed and finely chopped); 1/2 medium celeriac (peeled and chopped into small dice); 2 cloves garlic (finely diced); 1/2 teaspoon dried thyme; 1 teaspoon dried mixed herbs; 800 ml vegetable stock; pinch celery salt; 400 grams tin butter beans (or other white beans) (drained and rinsed); salt and pepper to taste; cold pressed flaxseed oil to garnish (optional); 2 tbsp chopped cooked chestnuts to garnish (optional)