Season the turkey leg on both sides with salt and pepper. Cut the onions, carrots, and celery into large chunks. Place the garlic clove, mustard seeds, bay leaf, ginger root, and juniper berries in a spice sachet or small cloth bundle.
Heat the oil in a large pan over medium-high heat and sear the turkey leg on both sides until browned, about 8-10 minutes total. Add the onions, carrots, celery, and tomato paste, and stew for 5 minutes. Deglaze with the white wine and let reduce for 2-3 minutes. Add the broth and spice sachet, bring to a boil, then reduce heat to low, cover with a lid, and braise for 2 hours.
Remove the turkey leg and spice sachet from the pan. Using an immersion blender, blend the vegetables into the sauce until smooth. Season with salt and pepper to taste. Remove the meat from the bone and serve with the sauce.