make the base
Add butter to a saucepan and place over low heat. When the butter melts, add the chopped onion or garlic and flour. Cook for about 5 - 7 minutes, stirring regularly with a whisk. The flour shouldn't turn brown. Gradually, slowly, start adding the vegetable stock (or water) while stirring continually. Bring to a boil and cook (low simmer) for 5 minutes, stirring occasionally.
Clean the wild garlic leaves under cold running water. Chop the wild garlic roughly and add it to the saucepan with the rest of the ingredients. Cook for 1 - 2 minutes for the wild garlic to cook slightly. Remove from the heat and blend using a blender or immersion blender. Optionally strain through a fine sieve for a silky smooth soup.
Season to taste with salt and pepper. Optionally stir in the heavy cream. Divide between four plates and serve.