1. Marinate the Chicken: Begin by combining the buttermilk and Cajun seasoning in a bowl. Add the chicken breasts, making sure they are fully submerged in the marinade. Cover and refrigerate for at least 1 hour, or up to 4 hours, to allow the flavors to infuse the chicken.

2. Prepare the Coating: In a separate bowl, mix together the all-purpose flour, paprika, garlic powder, onion powder, salt, and black pepper. This will be the coating that will give the chicken its signature crispy texture and Cajun-inspired flavor.

3. Coat and Fry the Chicken: Heat a generous amount of vegetable oil in a skillet over medium-high heat. Dredge the marinated chicken breasts in the flour mixture, making sure to shake off any excess. Carefully add the coated chicken to the hot oil and fry until golden brown and cooked through, about 6-8 minutes per side. Transfer the fried chicken to a paper towel-lined plate to drain any excess oil.

4. Make the Garlic Aioli: In a small bowl, whisk together the mayonnaise, minced garlic, lemon juice, salt, and black pepper. This creamy, garlicky aioli will provide the perfect cooling contrast to the spicy Cajun chicken.

5. Assemble the Sandwich: Toast the brioche buns to your desired level of crispiness. Spread a generous amount of the garlic aioli on the top and bottom buns. Layer the crispy Cajun chicken, followed by the fresh romaine lettuce and sliced tomatoes. Serve the Cajun Crispy Chicken Sandwich immediately, while the chicken is still hot and crispy.