Start by seasoning your brisket or chuck roast generously with Cajun seasoning, ensuring the spices coat the meat evenly. Let it marinate for at least 30 minutes at room temperature or refrigerate for up to 24 hours.

In a large skillet over medium-high heat, add a bit of oil and sear the marinated roast on all sides for about 4-5 minutes each side until browned. Transfer the roast to a slow cooker or Dutch oven.

In the same skillet, reduce heat to medium and add diced onions, bell peppers, and minced garlic. Sauté for about 5-7 minutes until softened and fragrant.

Combine the sautéed vegetables with beef broth and additional Cajun seasoning in the slow cooker or Dutch oven. Stir to ensure even distribution.

Cover and slow cook on low for 6-8 hours or high for 3-4 hours. If using a Dutch oven, cover and bake at 300°F (150°C) for about 3 hours.

Once cooked, remove the roast and let it rest for 10 minutes. Shred the beef into bite-sized pieces, discarding large chunks of fat.

If the gravy is too thin, transfer to a saucepan and simmer over medium heat for 5-10 minutes, stirring occasionally.

Toast the French bread until golden. Layer the shredded beef generously on the bread, top with gravy, lettuce, tomato slices, dill pickles, and mayonnaise.