Sprinkle the brisket with salt and black pepper. In a large ovenproof pot, such as a Dutch oven, heat up the oil on medium-low, and brown the brisket on both sides, about 5 minutes per side.
Remove the brisket from the pot and add the onions. While occasionally stirring, cook until they begin to brown. Add the garlic cloves and cook for another 2 minutes. Turn off the heat and pour into the pot the red wine vinegar, scraping along the bottom to loosen all of the pan drippings.
Return to the pot the brisket, fat-side up. Pour in the beef broth and add the cumin, jalapeños, cilantro and bay leaf. Cover the pot and place in the oven. Cook the brisket for 6 hours or until it’s fork tender. When you take the brisket out of the oven, let it rest in the pot uncovered for 30 minutes.
Meanwhile, to make the taco toppings, roast the Poblano chiles under the broiler until blackened, about 5 minutes per side. Place chiles in a paper sack or plastic food-storage bag, close it tight and let the chile steam for 20 minutes. Take the chiles out of the bag and rub off the skin. Remove stem and seeds and cut the chiles into strips. Heat up the vegetable oil in a skillet on medium low, and add the onion slivers. While occasionally stirring, cook until softened, about 10 minutes. Add the cut poblano strips and cook for 1 more minute.
After the brisket has rested, remove it from the pot, cut off the fat cap and shred the meat with two forks until it’s in long strands. To make the gravy, strain the cooled broth, throwing out the vegetables. Remove the fat from the broth with a gravy separator. Or alternatively, you can take a quart-sized plastic storage bag and pour some broth into it. Snip a bottom corner of the bag and drain the broth, stopping when you get to the fat layer that is on top. Add 2 tablespoons of the gravy to the shredded brisket, reserving the rest for serving. Taste the brisket and adjust seasonings.
To make the tacos, place on one side of each tortilla some Monterey Jack and slide the tortillas under the broiler for 30 seconds or until the cheese is melted. Fill the tortillas with shredded brisket and top with some of the onions and poblano strips. Serve with the pot juices and salsa.