In a heavy soup pot, heat the butter or oil and add the garlic. Soften, stirring frequently, then add the chopped greens and onions. Stir to coat and cook until they start to soften. If they stick to the bottom of the pot, add some stock. Add the stock and potatoes and simmer the mixture until the potatoes are cooked and just start to break down. Working in small batches, puree the soup in the bowl of your food processor or blender. Return the soup to the pot and season to taste with sea salt and fresh pepper, chopped fresh parsley or 1 teaspoon dried thyme. Serve the soup warm in heated soup plates. Garnish each bowl with a dollop of cream or sour cream. Makes six servings.