Kohlrabi Ale Soup

Name: Kohlrabi Ale Soup
Ingredients: 3 tablespoons butter; 1 leek, chopped; 1 carrot, chopped; 1 stalk celery, chopped; 1 clove garlic, chopped; 1 large kohrabi, peeled and chopped into large pieces; 1 bay leaf; 1 sprig thyme, or more to taste; 1 tablespoon flour; 4 cups vegetable or chicken stock; 1/2-1 cups beer (an ale works well; add what you like from the bottle and you can drink the rest); salt to taste; 1 tablespoon mustard, or to taste; fresh ground pepper; 1 cup grated cheddar cheese (optional but highly recommended)