Remove skin and all visible fat from duck legs and thighs; cut thighs off legs to make 8 pieces.
Place sage, orange zest, balsamic vinegar, grated ginger, allspice, coarse salt and pepper in mortar and grind with pestle to form a paste, add a little olive oil only if necessary.
Rub paste all over duck pieces; place in a zip lock bag, push air out and seal, refrigerate for 3 or 4 hours.
Heat oil in a large nonstick frypan, brown duck pieces on all sides, about 8 minutes.
Reserve any moisture and spices left in zip lock bag.
Remove duck to platter and keep warm.
Add to frypan, bay leaves, mushrooms, carrots, celery, sweet onion and sweet potatoes; saute until the veggies start to caramelize, 15 to 20 minutes on medium heat.
Add red wine to veggies and deglaze pan, add reserved orange juice; stir; slowly add buttermilk while stiring.
Add reserved juices and spices left in zip lock bag to pan and return duck pieces to pan with any accumulated juices.
Spoon veggies and juices over duck pieces, bring to boil, reduce heat and simmer covered for 45 minutes, remove cover, increase heat to medium high and reduce liquid until it starts to thicken slightly; about 15 minutes.
Adjust seasoning, add salt and pepper to taste.
(If you wish you may add a slurry of 1 teaspoon cornstarch and 2 tablespoons cold water; add to pan and bring to boil to thicken).
Remove duck pieces to platter; surround with veggies and drizzle with gravy.
Roll sage leaves and cut into strips, sprinkle over platter to garnish.