Heat oil in a large skillet over medium-high heat. Season flank steak with salt and pepper then sear in pan for 5-6 minutes on each side. Place steak in a 6-quart slow cooker sprayed with nonstick cooking spray.
Add all remaining birria ingredients to the slow cooker, then cover and cook on low for 8-10 hours.
After cooking, discard bay leaves and add steak to a large bowl and shred using two forks; set aside. Pour contents of the slow cooker into a blender (or use an immersion blender) and blend until smooth then pour back into the slow cooker to keep warm. Add 1⁄2 cup sauce to the shredded meat to keep from drying out.
Heat a skillet to medium-high heat and spray with nonstick cooking spray. To assemble the tacos, wrap a few tortillas at a time in a damp paper towel and microwave for 30 seconds.
Top each softened tortilla with 30 grams of meat and 7 grams of cheese. Fold in half, ladle or brush some sauce from the slow cooker over the top and cook for a few minutes on each side in preheated skillet. Serve tacos with extra birria sauce for dipping.