2 tablespoons (30ml) extra-virgin olive oil, plus more as needed and to serve
1 small onion or half a large onion, finely diced (about 8 ounces; 225g)
3 medium ribs celery, finely diced (about 6 ounces; 170g)
3 medium cloves garlic, minced (about 3/4 ounce; 25g)
1 1/2 pounds (700g) zucchini, split lengthwise and cut into 1/2-inch disks
1 cup packed fresh basil leaves
3 cups (1.2L) water or broth
Heat olive oil in a large saucepan over medium heat until shimmering. Add leek and celery. Season with salt and cook, stirring, until vegetables are softened but not browned, about 5 minutes. Add garlic and cook, stirring, until fragrant, about 30 seconds. Add zucchini and cook, stirring, for 5 minutes. Add water or broth and bring to a boil.
Bring to a simmer and cook, stirring occasionally, until zucchini is tender, about 10 minutes. Add basil and blend soup using a handheld immersion blender or a countertop blender until it is as rough or as smooth as you like it.