Preheat oven to 300
Remove the beef from the packaging and strain the juices. Add the meat to a baking dish. Reserve the seasoning packet
Add everything except the meat and the cabbage to a medium sauce pot. Bring to a simmer and cook about 10 minutes. Remove from heat and allow to cool for 5 minutes.
Pour into the blender. Be careful at first when blending hot liquid. I like to keep the hole in the cover open and hold a clean kitchen towel over it. Pulse first, then blend on high until smooth.
Pour the liquid over the meat. Flip the meat to make sure it is fully coated on all sides with the sauce. Cover and bake for 3 hours.
Chop the cabbage into quarters splitting the core. Cut the core off each quarter. Then cut each quarter in half. Remove the meat from the oven and add the cabbage. Tuck the cabbage into the pan so the liquids are covering it. Cover the pan again and bake 1 more hour.
Remove the pan from the oven and allow to cool for 10 minutes. Take the beef out carefully, letting as much sauce as possible drip off. Place on a cutting board.
Take the cabbage out with a slotted spoon, again letting as much sauce drip off as possible, and transfer to another cutting board.
Slice the corned beef into 1/2 inch thick slices. Chop the cabbage roughly.
Heat a griddle to medium high heat with some oil. Add the brisket and cook to slightly brown on all sides. As it cooks, break it up a bit. Add in some of the cabbage and cook more. You can add as much cabbage as you personally want, I only added about half of it. Add a few tablespoons of the birria sauce.
Slide the mixture to a cooler side of the griddle and start making the tacos!
Add a tablespoon of the birria sauce and top with a tortilla. I was able to do 2 at a time on my griddle.
Add a little more of the sauce to the top of the tortilla and flip it. Add about 1 ounce of cheese, then a scoop of the meat and cabbage mixture. Fold the taco over and press lightly to flatten. Make sure there is plenty of oil on the griddle, and continue to cook until the cheese is melted and the edges are crispy. the whole tortilla should be crispy and browned in some areas and blackened in spots. There should also be crispy cheese around the edge. Transfer this to a wire rack and you can either give it to the first person to start eating, or transfer to a wire rack and pop into the oven at 170 and continue until they are all done.
Warm the remaining birria sauce (consume) in the microwave and split into individual bowls for each person. Top each little bowl with chopped cilantro and diced onions. Serve.