Rough chop the carrots, onion, and garlic. Place in dutch oven with brisket, full bottle of wine, and the beef broth. Also, add the juniper berries, cloves, thyme, rosemary, bay leaves, and sage. Refrigerate overnight or at least 12 hours turning every so often to achieve even marination.
Next day, strain the liquid and separate the veggies and brisket. Dry the brisket very well to achieve a good sear. Season the brisket aggressively on all sides with 1-2 Tbsp of kosher salt and a little bit of black pepper.
Heat dutch oven on medium-high heat and add 1/4 cup of olive oil. Brown the brisket on all sides. Should take 20-25 minutes to get a nice sear on all sides. When finished with brisket, set aside and saute the strained veggies and herbs for 10-15 mediums on medium.
Add the liquid back to the pot and scrape the bottom with a wooden spoon to remove the flavor bits. Add brisket in and cover. Cook in oven for 3-4 hours turning every hour.
When finished remove meat and tent with foil. Separate the veggies and liquid again. Remove and discard the herbs. Add the veggies back to the pot and boil sauce to thicken for 20 minutes. The sauce should coat the back of a wooden spoon, but not be overly thick.
Blend the sauce with an immersion or regular blender to further thicken it. Add the cognac and cook for 1 more minute. Add the butter and stir to finish the sauce.
If the sauce is not thick enough mix 1 Tbsp of cornstarch and 2 ounces of water thoroughly and add to sauce. Boil out for 1 minute to thicken. The sauce should not be overly thick and this step is probably not necessary.
Cut the brisket against the grain and spoon the sauce all over the meat in a serving platter. Serve with polenta or mashed potatoes. Enjoy!