Cheesy cauliflower flask soup

Name: Cheesy cauliflower flask soup
Ingredients: 1 cauliflower (about 1kg); 1 tbsp olive oil; 1 onion, finely chopped; 2 celery sticks, finely chopped; 1 garlic clove, crushed; small handful of thyme; 1 large potato (about 340g), cut into chunks; 500ml milk; 600ml vegetable stock; 100g mature cheddar, grated; 200g sourdough or other crusty bread, torn into bite-sized chunks; 1 tsp dried mixed herbs; 1 tsp garlic granules; 25g parmesan or vegetarian alternative, finely grated; 2 tbsp olive oil