Bring ¾ cup (185 milliliters) white wine to a simmer. Let it simmer just long enough to evaporate the alcohol, then add a small handful (about 4 teaspoons/20 milliliters) coarsely chopped tarragon stems. Cover the pan and let the stems infuse in the hot wine for 10 to 15 minutes. Add 1 cup (250 milliliters) demi-glace to the infusion, and reduce the mixture by one-third. Strain through a fine chinois. Finish the sauce with 2 teaspoons (10 milliliters) tarragon leaves that have been chopped at the last minute.