Season the brisket generously with salt and pepper.
In a large cast iron skillet, over medium-high heat, heat the olive oil and, once smoking hot, brown the brisket on all sides until golden brown.
Transfer the browned brisket to your slow cooker, fat side up.
Back in the skillet, lower the heat to medium low and add some more olive oil if necessary. Add the onions and garlic and sauté for about 10 minutes or until the onions have caramelized. Transfer to the slow cooker.
In a medium sized bowl, combine the blended tomatoes, the Worcestershire sauce, the vinegar, the mustard, the brown sugar, the ketchup and the chili powder, whisking everything together until smooth. Pour the sauce into the brisket in the slow cooker.
Finally, add a whole chipotle pepper to the slow cooker and cover. Cook on low for 9 hours, until the brisket is fork-tender.
Discard the chipotle pepper and take the brisket out of the slow cooker.
On a cutting board, take the fat out using a serrated knife. Discard the fat.
Shred the brisket using two forks and bring it back to the slow cooker. Cover and cook for one more hour.
If you don't have a slow cooker, just braise your brisket in a dutch oven in the oven (300F degrees) for 3 to 4 hours!
For the White BBQ Sauce:
¾ cup mayonnaise
2 tablespoon water
2 tablespoon apple cider vinegar
½ tsp salt
2 tsp coarsely ground pepper
1½ tsp mustard
½ tsp sugar
2 garlic clove, minced
1 tsp prepared horseradish
Combine all ingredients in a blender and blend until smooth. Add more mayo if you want a thicker consistency.