Start by rinsing and de-stemming the tomatoes. Roast them in a 400F oven for 20-25 minutes or until you need them.
Wipe off any dusty crevasses on the dried chiles. De-stem and de-seed the chiles, but don't worry about getting rid of every last seed. Roast them in the oven for 1-2 minutes or until warm and fragrant. Add the chile pieces to a bowl and cover them with the hottest tap water you've got. Let them reconstitute for 20 minutes or so.
Roughly chop 1 onion and peel 6 garlic cloves. Add a glug of oil to a skillet on medium heat and saute the onions and whole garlic cloves. Once the onion has softened and lightly browned you can add this mixture to the blender.
Add a thin layer of oil to a skillet and preheat to medium-high. Chop up the chuck roast into 1-2" chunks and give it a good salting. Sear each side of the beef in the skillet for a few minutes or until it is browning. Add the seared meat pieces to the slow cooker. You can optionally deglaze the pan with the 2 cups of stock that's used to liquefy the sauce.
Before draining the reconstituted chiles take a taste of the soaking liquid. If it tastes bitter to you then use stock for the sauce. If you like the flavor you are welcome to use the soaking liquid in place of the stock.
Add the drained chiles, roasted tomatoes, and the onion-garlic mixture to a blender along with: 2 cups of stock (or what you used to deglaze the meat pan), 1 teaspoon cumin, 2 teaspoons Mexican oregano, 1/8 teaspoon cinnamon, pinch of ground clove, 2 teaspoons salt, and some freshly cracked black pepper. Note that you may not need to add as much salt as me if using store-bought stock. Combine well.
Cover the seared meat pieces with the sauce. Slow cook on low for 4-6 hours. Alternatively, you can simmer the beef on the stovetop, covered, for 2-3 hours or until it is fall-apart tender.
Once cooked you can optionally skim off any fat that has risen to the surface -- this fat can be used to crisp up the tacos. It's also a good time to take a taste of the sauce and adjust it to your liking. I like having a strong hint of clove and cinnamon in this recipe so I added another pinch of each.
Shred the beef using two forks and discard any fatty chunks that you don't want to eat. Add the shredded beef (or as much as you are using for tonight's meal) to a separate bowl and add enough sauce to give it a thorough coating. Adding the sauce to the shredded beef is the key so don't skip this step!
If you'd like to make traditional Birria tacos, the key is to start crisping up the corn tortillas in some fat BEFORE adding the Birria. I used lard, but you could also use olive oil or even the fat that has risen to the surface of your stewing pot. Simply add a thin layer of fat to a skillet over medium heat. Once the fat is heated you can add as many tortillas as the skillet will fit, giving them a minute or so to start crisping up. Then add Birria and cheese to each tortilla, fold them half, and crisp up each side of the tacos until brown spots are forming on the tortillas.
Serve the tacos immediately with your choice of fixings. I added raw onion and freshly chopped cilantro to the insides of the tacos, and served them next to plenty of the Birria sauce for additional dipping and drizzling. And don't forget a final squeeze of lime!
Store leftover Birria in an airtight container in the fridge where it will keep for a few days.