Ina sauté pan, melt the butter and sauté the onions until soft. Peel, core, and thinly slice the apples. Add to the onions, and sauté until the apples are soft, about 3 to 5 minutes more. Remove from the heat and add the vinegar and salt. Refrigerate until cold.
Cider Sauce:
Ina medium saucepan, simmer the sparkling cider until only 1/2 cup remains, 30 to 45 minutes. Whisk in the brown gravy; simmer gently for 10 minutes. Remove from heat.