1. Preheat oven to 350°.
2. Weigh or lightly spoon flour into dry measuring cup; level with a knife. Place flour in a shallow dish. Sprinkle beef evenly on all sides with salt and pepper; dredge in flour. Heat a large Dutch oven over medium-high heat. Add oil to pan, and swirl to coat. Add beef to pan; sauté for 8 minutes, turning to brown on all sides. Remove beef from pan.
3. Add diced onion, diced celery, diced carrot, and chopped leek to pan; sauté 5 minutes, stirring occasionally. Add garlic; sauté 1 minute, stirring frequently. Place peppercorns, juniper berries, 3 thyme sprigs, and bay leaves on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely. Place herb and spice bundle in pan; return beef to pan. Dissolve demi-glace in 3/4 cup hot water, stirring well. Add demi-glace mixture and wine to pan; bring to a boil. Cover and bake at 350° for 2 1/2 hours or until beef is fork tender. Remove beef from pan; strain cooking liquid through a fine-mesh sieve over a bowl. Discard solids. Serve cooking liquid with beef. Garnish with thyme sprigs, if desired.
Wine note: With the richness of short ribs, you need to pull out a cabernet as an accompaniment. (Well-marbled beef loves to tame this grape's tannins.) Slingshot 2006 Cabernet Sauvignon is a Napa bottle that, at $23, won't break the bank. But it offers beautiful nuances of cedar, spice (juniper's a big player), and aromatic dried cherries, with bright acidity. —Sara Schneider