Preheat the oven to 180 degrees. Peel the asparagus and place on a baking sheet. Clean the pak choi and distribute the individual leaves between the asparagus. Spread the tarragon leaves and stems on top. Sprinkle with sugar and salt. Pour in the white wine and top with flakes of butter. Cover the baking sheet as tightly as possible with aluminum foil.
Cook in the oven for about 45 minutes. A great tarragon sauce can be made from the leaked juice.