To marinade the meat add all marinade ingredients to a large bowl. Mix well and cover, refrigerate overnight, or up to 2 days.
To a Dutch oven, heat the vegetable oil over medium heat. With a slotted spoon carefully add the meat to the oil, saving the marinade for later. Brown the beef, searing each side, and add the salt and pepper while cooking. Place the cooked browned meat in a separate bowl and set aside. Leave any juice in the bottom of the Dutch oven.
To the Dutch oven, add the wine marinade, water, the spice sachet, the demi glace, and the mushrooms. Boil down for about 20 minutes until has reduced. Add the browned meat back to the pot, and cover with a lid. Place the pot in a 375 degrees Fahrenheit oven for 2.5 hours until the meat is fork tender and falls apart.
Transfer the pot back to the stove top. Remove the meat again from the pot, and discard the sachet, leaving the broth in the bottom of the pot. Over medium heat, reduce the sauce down again to about 4 cups.
In a separate sauce pan, melt the butter and stir in the flour to make a roux. Slowly whisk the roux into the Dutch oven sauce, and continue stirring until the sauce thickens. Add the meat back into the sauce to heat the meat all the way through. Serve with au gratin potatoes and enjoy!