Combine the seasoning mix ingredients in a small bowl.

Melt 2 tablespoons of the butter in an 8-inch skillet over high heat. When it sizzles, add the onions, celery, 2 teaspoons of the seasoning mix and 1 teaspoon of the ginger. Cook, stirring once or twice, until the vegetables are browned, about 6 to 7 minutes. Remove from the heat and let cool.

Preheat the oven to 350°.

With a small, sharp knife, make slits about 1 inch apart in the top of the roast, taking care not to cut all the way through. Stuff the holes with sprigs of fresh dill pulled from 2 of the branches (reserve 1 large sprig for garnish). Sprinkle 1 tbsp plus 1 teaspoon of the seasoning mix evenly over the meat and rub it in well. Spread the vegetable mixture over the top and sides of the roast. Place the meat in a roasting pan and roast until it is fork tender and juicy, about 2-1/4 hours.

Pour off the drippings from the pan and cover the roast to keep it warm. Skim off 1/3 cup of fat from the drippings and combine it with the remaining 2 tablespoons butter in a 10-inch skillet over high heat. When the butter melts, whisk in the flour, Add the garlic and 2 teaspoons of the ginger and cook, whisking constantly for 2 minutes. Add the minced fresh dill and 1-1/2 cups of the stock and cook, whisking for 2 minutes. Add 1 cup of the cream and cook, whisking, for 5 minutes. Add the remaining seasoning mix and cook, whisking, for 2 minutes. Add the remaining cream, ginger, and stock, and cook for 5 minutes, whisking constantly. Remove from the heat.

To serve, slice the roast, drizzle the sauce over it, and garnish with the reserved sprig of fresh dill.