Step 1

Melt oil and butter together in a large saucepan over low heat. Add leek and 1/2 tsp salt flakes, and cook, stirring, for 25 minutes or until very soft. Add garlic and rosemary, and cook, stirring, for a further 5 minutes or until softened and fragrant.

Step 2

Add potato, zucchini and stock with bay leaf and parmesan rind, and bring to a simmer. Cook for 30 minutes or until potato is very tender. Fold through parmesan, then add spinach and parsley. Remove from the heat and discard parmesan rind.

Step 3

In two batches, transfer soup to a blender and whiz until smooth. Season to taste. Combine sour cream with 1 tbs cold water, to loosen.

Step 4

Divide soup among bowls and drizzle with sour cream. Scatter with croutons, then top with chives, dill and freshly ground black pepper, to serve.