Melt butter (or heat up oil) on medium heat and add chopped onions- cook until translucent and starting to become golden- about 5-6 minutes. If you want your soup to be white-ish, peel off courgettes, if you are not bothered, keep the skin on. Add slices of courgette, chopped celery sticks and cook until they start to brown a little. Add clove of garlic and deglaze with wine. Cook to reduce the liquid and get rid of alcohol. Add potatoes, stock cube and just enough water to cover all the vegetables. Simmer for about 15 minutes until all vegetables are soft. Blend using hand blender and serve hot.
If you have some leftover cream that needs using, add it to the soup, otherwise I wouldn't bother; It's perfect on its own.
Notes
It will keep in the fridge for about 3 days or freeze it for up to 3 months. It reheats really well.