Chop the zucchini crosswise, then lengthwise and then roughly into pieces. With the gas on medium heat, heat the oil in the pan of a pressure cooker and add the garlic, zest and chilli flakes. As the garlic starts to sizzle and the oil becomes fragrant, add the chopped zucchini. Give it a good stir, allowing the zucchini to get coated with the other ingredients. Pour the vegetable stock over the zucchini and add a pinch of salt. Cover the pressure cooker with its lid and seal tightly. Turn the gas flame on to high and give it just one whistle. Allow the steam to be let off completely before opening the lid of the pressure cooker. Allow the contents of the pressure cooker to cool, then blend either with a hand blender or in a mixer until smooth. Check for salt and season with salt and pepper as required.