1. Heat your oven to 190°C.
2. Place the potatoes pieces into a large saucepan and cover with cold water. Bring to a simmer and cook for 15 minutes. Drain and return the potatoes in the pan to low heat for a further minute to remove some additional moisture. Add the olive oil to the potatoes and stir gently to coat,then transfer to a baking sheet and season with salt.
3. Roast potato in the oven for 45 minutes to an hour,turning occasionally,until the potato croutons are golden brown and crisp.
4. In the same saucepan as you used earlier,heat a little of the butter and fry the mushrooms,onion and in batches until well browned,then remove them. Add the remaining butter to the pan,then the flour. Cook for 2 minutes,stirring to form a thick roux,then add the Vegemite (if using) and tomato paste and cook for a further minute. Add the vegetable stock or water a little at a time,stirring to avoid lumps forming,then add the mushroom,garlic and onion mixture.
5. Simmer for 15 minutes,then blend lightly with a stick blender to form a very coarse,thick soup. Stir the white wine vinegar and spinach leaves into the hot soup and serve with the croutons and a drizzle of olive oil.
Suggestion: Now that you've been very good,try Adam's peanut butter panna cotta for dessert!