How to Make It
In a saucepan, combine the stock with the milk, thyme and bay leaves. Cover and bring to a simmer over low heat.
In a large saucepan, heat the vegetable oil. Add the potatoes and leeks and season lightly with salt. Cover and cook over moderately low heat until the vegetables are softened, about 7 minutes. Pour the stock mixture over the potatoes and leeks, cover and simmer over moderately low heat until the potatoes are tender, about 15 minutes. Discard the thyme and bay leaves.
Bring the soup to a boil. Add the spinach and wilt over moderately high heat, 1 minute. Working in batches, puree the soup in a blender. Return the soup to the saucepan and season with salt and pepper.
Ladle the soup into bowls. Scoop a rounded tablespoon of the Horseradish Granité onto each toast. Float the toasts in the soup and serve right away.