- Preheat the oven to 190C/gas 5.
Add 2 tsp of the oil to a saucepan, then add the onions and cook over a very low heat for 30–40 minutes, stirring regularly so that the onions caramelise.
- While the onions are caramelising, put the remaining oil onto a baking sheet. Add the potatoes and muddle around until they are coated, then bake in the oven for 30–40 minutes.
- Add the stock to the onions, bring to the boil and simmer for 5 minutes.
- Meanwhile, to make the cheesy croutons: Cut each slice of bread in half and grill, then turn over and top each with the Cheddar cheese and mozzarella, and grill again until the cheese melts and browns. Alternatively, you can put the cheese-topped bread on a baking sheet and cook in the oven for 5-10 minutes.
- Stir the parsley, pepper and soy sauce through the soup, then place into four bowls. Serve with cheesy croutons and a sprinkling of fresh parsley on top, and the potato wedges on the side.