Preheat oven to 325°F.

In a dutch oven warm oil over high heat. Sear the brisket on all sides.

Put garlic, chilies with sauce, tomatoes, vinegar, and lime juice in a blender. Blend until mixture is completely smooth.

Add chili mixture to the dutch oven and then add enough stock to completely cover beef. Bake in the oven for 2 hours or until brisket is extremely tender. If needed, add more stock or water occasionally making sure beef is covered.

Pull the dish from the oven and remove meat. Reserve about a quarter of the juices. Shred the beef using forks.

Add beef back to juices and toss to coat.

Serve on tortilla and top with onions and lettuce.