In a medium pot, heat the oil and butter over medium-high heat. When the foam subsides, add the onion and cook for a few minutes, until soft. Add the garlic and cook for another minute, then add the zucchini and cook for a few minutes, until it starts to soften.
Add the stock, bring to a simmer, reduce the heat and cook for about 20 minutes, until the zucchini is very soft. Puree with a hand-held immersion blender or in batches in a regular blender (careful if it's hot!) and season to taste with salt and pepper. (If you like, add a good glug of cream.)