For the marinade: Combine the Maggi sauce, lime juice, 2 tablespoons water, garlic and serranos in a medium-size bowl, season with salt and pepper. While whisking constantly, slowly add the olive oil, Put the brisket pieces into a large food-safe plastic bag and pour the marinade over it, turning the meat to coat. Seal and refrigerate for at least 8 hours or preferably overnight.
Allow the meat to come to room temperature. Drain the meat and discard the marinade.
For the brisket: Pour the oil into the bottom of a slow cooker. Add the onions, broth, peppers, oregano, garlic, chiles, tomatoes and some salt and pepper. Mix well, and then nestle the meat into the liquid in the bottom of the slow cooker. Cook the brisket on medium-high heat until the meat is tender and shreds easily with a fork, about 5 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 2-inch-long pieces. Return the shredded meat to the slow cooker, stir to combine, and let stand until the meat is hot, about 10 minutes. Taste and season with salt and pepper, if necessary.
The warm braised meat can be used to fill burritos, tacos, tostadas or sandwiches. Leftover meat can be stored in an airtight container for up to 3 days or frozen in plastic freezer bags or containers for up to 1 month.
For the tostadas: Spread about 1/3 cup of the warm meat over each of the crisp shells. Top each tostada with about 2 tablespoons lettuce, some diced avocado and 2 tablespoons salsa. Drizzle about 1 tablespoon crema over the top and garnish with crumbled queso fresco.
Serve with lime wedges on the side for squeezing over the top.