In a small bowl, combine salt, pepper, chili powder and garlic. Sprinkle the mixture evenly over the brisket, making sure to apply a coating over the entire piece of meat. Place the brisket in a slow cooker and pour Dr. Pepper over the top and cook on low for 8 hours. Remove from the slow cooker and let rest for 15 minutes. Slice into small pieces against the grain.
To make the barbecue sauce, heat the olive oil in a sauce pan over medium heat. Cook the onions until just soft, about 5 minutes. Add the garlic and saute until fragrant, about 1 minute. Stir in ketchup, lemon juice, Worcestershire sauce, mustard, mango chutney and salt and pepper to taste, simmer for 5 minutes. Let cool slightly then process in a blender until smooth.
To assemble the quesadillas, heat a large skillet over medium heat until hot, about 2 minutes. Place 1 tortilla in skillet and toast until soft and puffed slightly at edges, about 2 minutes. Flip tortilla and toast until puffed and slightly browned, 1 to 2 minutes longer. Slip tortilla onto cutting board. Repeat with remaining tortillas while assembling first quesadilla. Divide meat, brie and Monterey Jack Cheese over half of tortilla, leaving 1/2-inch border around edge. Top with a tablespoon of the barbecue sauce.Fold tortilla in half and press to flatten. Brush surface generously with oil, sprinkle lightly with kosher salt, and set aside. Repeat to form remaining quesadillas.
Working two quesadillas at a time, place in skillet, oiled sides down with folded edges toward center of skillet.; cook over medium heat until crisp and well browned, 1 to 2 minutes. Brush surfaces with oil and sprinkle lightly with salt. Flip quesadillas and cook until second sides are crisp and browned, 1 to 2 minutes longer. Transfer quesadillas to cutting board; cool about 2 minutes, halve each quesadilla, and serve. Serve with remaining barbecue sauce.