An intriguing combination of flavors combine here in a fresh-tasting, healthy soup that defies easy recognition. Tarragon works equally as well as mint as a garnish, if you prefer.
Melt the butter in a large saucepan over medium heat, then add the onions. Cook slowly, stirring frequently, until the onions soften and turn lightly golden.
Meanwhile, peel and core one of the apples, and cut it into 1-inch slices. Reserve the remaining apple until serving.
Add the apple slices, celery, tomato, wine, celery seeds, nutmeg and ginger to the pan, and place a round of parchment or wax paper directly atop the vegetable mixture. Cover the pan, reduce the heat to low and simmer slowly until the vegetables are very soft, about 1 hour.
Uncover the pan, discard the paper, and add the stock to the soup. Purée the soup until smooth in a blender or food processor. Strain the soup through a fine mesh sieve if you prefer a more silken texture. Season to taste with salt and pepper.
Just before serving, cut the reserved, unpeeled apple into a tiny dice. Reheat the soup gently and divide among heated bowls. Garnish each serving with a dollop of mascarpone, if using, and a sprinkle of diced apple and julienne mint.