In a large bowl whisk together 1 tablespoon olive oil, dijon mustard, lemon juice, garlic powder, cumin, salt, and pepper.
Using a meat mallet, pound the chicken breasts to an even ½-inch thickness. Add the chicken to the bowl with the marinade and turn to coat. Marinate for at least 1 hour.
Meanwhile, make the sauce by whisking together mayonnaise, dijon mustard, apple cider vinegar, smoked paprika, garlic powder, salt, and pepper. Set aside.
Preheat grill to medium-high heat. Brush grill grates with remaining tablespoon of olive oil and grill the chicken for about 5-7 minutes on each side.
To assemble the sandwiches, spread the sauce on each half of the brioche buns place a piece of chicken on the bottom half. Top with lettuce and tomatoes, then add the top half of the bun.