Step 1Heat the oven to 325 degrees. Season the brisket all over with salt to taste, cumin and oregano.
Step 2Lay the onion into a baking dish large enough to hold the meat. Add the peppercorns and bay leaf. Lay the brisket on top. Pour the beef broth around it. Cover the pan and bake until the meat is very tender and shreds easily, 2 1/2 to 3 hours. Remove from the pan to a cutting board and let stand until the meat is cool enough to handle.
Step 3Combine the juice from 2 1/2 limes with the chipotles and corn oil in a bowl and whisk to blend.
Step 4Cut the brisket crosswise into strips 1 inch wide. Remove the extra fat. Using two forks, or your fingers, separate the meat into thin shreds. Place in a large bowl and pour the lime mixture over the meat. Toss to coat. Add the jalapenos, cucumber, cheese, green onions, cilantro and tomato. Toss to combine. Season with salt to taste. Refrigerate if not serving the salpic?n immediately.
Step 5Just before serving, gently mix in the diced avocados. Season to taste, and add additional lime juice if desired. Serve in a large bowl lined with lettuce leaves.