Chunky cheddar & celeriac soup

Name: Chunky cheddar & celeriac soup
Prep time: 15 min
Cook time: 40 min
Total time: 55 min
Cuisine: British
Ingredients: 1 tbsp butter; 3 onions , finely sliced; 500g floury potatoes , peeled and diced; 2 celeriac (1kg/2lbs 4oz in total) peeled and diced; 1l chicken or vegetable stock (from a cube is fine); 4 sage leaves; 2 strips lemon peel; 200g mature cheddar , diced; crispy sage leaves, optional