Brown the brisket on all sides in a Dutch oven. Remove.
Heat 1/4 cup oil in Dutch over medium heat.
Gradually whisk in the flour. Keep whisking until smooth, adding more oil if needed.
Cook roux 15-20 minutes or until dark brown, stirring constantly with wooden spoon. Do not burn.
Meanwhile, warm the beef broth.
Slowly (WARNING: Be very careful! The roux will be very hot and the broth will boil when added. Keep your hands and face away from over the pot) pour the broth into the roux while whisking vigorously to break up any clumps.
Bring to boil, reduce to medium low and simmer 20 minutes, skimming away any foam or fat.
Scrape bottom to loosen any settled roux.
Pour into slow cooker. Set heat on low.
Heat 3 additional tablespoons of oil in the Dutch oven over medium-high.
Add the onion. Saute 15 minutes.
Deglaze with 1-2 tablespoons of water if bottom of pot becomes too brown.
Add garlic, bell pepper, jalapenos, and Cajun seasoning. Saute 3 minutes. Pour into slow cooker with broth mix.
Add bay leaves, cover and simmer on low for 4-5 hours or until meat starts to fall apart
Remove meat to a cutting board and chop as desired. Return to cooker and keep warm until ready to serve.