In a large stockpot, heat two tablespoons of olive oil over low heat. Add finely chopped onions and cover the pot, allowing them to sweat for about 3-5 minutes, stirring occasionally.

Once the onions are soft, stir in the diced zucchini and peeled russet potatoes. Pour in four cups of chicken stock or vegetable stock, ensuring the vegetables are submerged. Cover and bring to a gentle simmer for 15-20 minutes.

Remove the pot from heat and add roughly chopped fresh basil. Using an immersion blender, blend the soup until smooth and creamy.

Taste and season with salt and freshly ground black pepper. Stir well to incorporate the seasoning.

Ladle the creamy soup into bowls, adding a dollop of crème fraîche on top if desired, then serve warm or chilled.