Broccoli and cheese soup

Name: Broccoli and cheese soup
Ingredients: 1 brown onion, roughly chopped; 1 leek, roughly chopped; 1 large garlic clove, crushed; 1 tbsp olive oil; 25g/1oz butter; 100g/3½oz potato, peeled and diced; 400g/14oz broccoli, stalk chopped, florets cut into 2cm/¾in pieces; 1 litre/1¾ pints vegetable stock (or light chicken stock); 1 bay leaf; salt and freshly ground black pepper; 150ml/5fl oz full-fat milk; 150g/5½oz mature cheddar, grated; 100g/3½oz crumbly blue cheese (such as Stilton); sourdough croûtons