BBQ Braised Brisket Sandwiches

Name: BBQ Braised Brisket Sandwiches
Ingredients: 1 tablespoon chile flakes; 1 tablespoon chili powder; 1 tablespoon garlic powder; 1 tablespoon onion powder; 1 tablespoon paprika; 1 tablespoon packed light brown sugar; Kosher salt and freshly ground black pepper; One 3-pound brisket; 2 tablespoons olive oil; 2 cups Nancy's Sweet and Tangy BBQ Sauce, recipe follows; 1 tablespoon Worcestershire sauce; One 12-ounce bottle lager beer or chicken broth; 6 ciabatta sandwich rolls; Mayonnaise, for serving; 2 tablespoons butter; 1/2 medium red onion, chopped; 3 cloves garlic, minced; 2 cups tomato sauce; 1 cup ketchup; 1/2 cup packed light brown sugar; 1/4 cup white wine vinegar; 2 teaspoons dry mustard powder; 1/2 teaspoon cayenne pepper, plus more if needed; Kosher salt