Set out a large 6-8 quart slow cooker. Preheat on high. Set a skillet over medium heat, and add the oil.
Veggie Prep: Peel and slice the onions first. Then place them in the skillet. Sauté for 2-3 minutes to soften. Meanwhile, cut the mushrooms in half. Then cut the potatoes in half, and slice into ¼ - 1/3 inch pieces.
Place the potatoes in the bottom of the slow cooker. Add in the mushrooms and sautéed onions. Drizzle the red wine vinegar over the top.
Brisket Prep: Massage the beef base over the surface of the brisket. In a small bowl mix the brown sugar, smoked paprika, salt, pepper, garlic powder, and onion powder. (If using powdered bouillon instead of beef base, add it to the dry rub.) Sprinkle and rub the brisket with the spice blend on both sides.
Lay the brisket over the chopped produce in the slow cooker. Cover and cook on high for 6-7 hours, or on low for 11-13 hours.
When ready to eat, move the brisket to a cutting board. Cut into thin strips against the grain. Then place the potato mixture on a serving platter and arrange the sliced brisket on top. Garnish with parsley if desired. Serve warm.
If you don’t love mushrooms, skip them and add in another potato or two.You can sear the brisket in a skillet, to create a crust, before adding to the crock pot, but it is not necessary. If you decide to do so, only sear it 1 minute per side, as to not burn the garlic and onion powder.