Pat the chicken dry and slice each breast into two thinner cutlets.
2 boneless, skinless chicken breasts
Whisk the olive oil, lemon juice, garlic, salt, thyme, and pepper together in a large bowl. Add the chicken and toss to coat.
¼ cup extra virgin olive oil, 2 tablespoons lemon juice, 2 cloves garlic, 1¼ teaspoons kosher salt, 1 teaspoon dried thyme, ½ teaspoon freshly ground black pepper
Cover and marinate in the refrigerator for 2 hours and up to 24 hours.
In a small bowl, stir the mayonnaise, barbecue sauce, and dijon mustard together. Refrigerate until ready to use.
½ cup mayonnaise, ¼ cup barbecue sauce, 1 tablespoon Dijon mustard
Preheat a grill or grill pan over medium-high heat. Brush the grill with oil.
Grill the chicken until cooked through, about 3 minutes per side. During the last minute of cooking, add a slice of cheese to each piece of chicken. Cover and cook until the cheese is melted. Remove from the grill and set aside.
8 slices provolone cheese
Brush the grill with more oil and grill the onion rings until grill marks appear and the onions are cooked through, about 4 minutes per side.
1 red onion
Assemble the sandwiches: spread each side of the bun with the sauce. Top with lettuce, grilled chicken, bacon and grilled onions. Serve immediately.
4 brioche buns, 4 pieces lettuce, 4 slices cooked bacon