Prepare the brisket: Whisk the lime juice, Maggi seasoning, 2 tablespoons water, half of the garlic and 1 serrano in a medium bowl; season with salt and pepper. Slowly pour in the olive oil, whisking constantly. Place the brisket pieces in a large resealable plastic bag and add the marinade; turn the meat to coat. Refrigerate at least 8 hours or preferably overnight.
Bring the meat to room temperature, about 45 minutes. Remove from the bag and discard the marinade.
Pour the vegetable oil into the bottom of a 6- to 7-quart slow cooker. Add the onion, bell pepper, beef broth, tomatoes (and their juices), oregano, the remaining garlic and 2 serranos, 1 tablespoon salt and a few grinds of pepper. Mix well, then nestle the meat in the vegetable mixture. Cover and cook on high until the meat is tender and shreds easily with a fork, about 8 hours.
Using a slotted spoon, transfer the meat to a cutting board and let stand until cool enough to handle, about 15 minutes. Shred the meat into pieces. Return the shredded meat to the slow cooker, stir to combine and let stand until the meat is hot, about 10 minutes. Season with salt and pepper, if necessary.
Assemble the tostadas: Spread about 1/3 cup of the warm meat on each tostada shell. Top with lettuce, avocado, salsa and queso fresco. Serve with crema and lime wedges.