Step 1
Place chicken breasts in a shallow dish. Coat with Italian dressing. Cover with plastic wrap and let marinate in the refrigerator 8 hours to overnight.
Step 2
Heat a grill pan over medium-high heat. Remove chicken breasts from marinade and place in the skillet; cook until no longer pink in the center and the juices run clear about 6 minutes per side.
Step 3
Slice chicken breasts; lay Brie cheese on top to melt.
Step 4
Preheat oven to 300 degrees F (150 degrees C).
Step 5
Heat 1 teaspoon olive oil in an oven-safe skillet over medium heat. Add tomatoes and 1/8 teaspoon salt; cook stirring once until tomatoes release their juices about 4 minutes. Remove from heat and stir in 2 tablespoons balsamic vinegar and basil.
Step 6
Transfer skillet to the preheated oven; bake tomatoes until softened about 15 minutes.
Step 7
Combine remaining 2 teaspoons olive oil 1 tablespoon balsamic vinegar and 1/8 teaspoon salt in a bowl. Add spinach; toss to coat.
Step 8
Mix mayonnaise Dijon mustard garlic and pepper together in a bowl.
Step 9
Place bottom half of the loaf on a baking sheet. Spread mayonnaise mixture on top. Cover with spinach and tomatoes. Place chicken with Brie cheese on top. Cover with top half of the loaf.
Step 9
Bake in the preheated oven until heated through 5 to 10 minutes. Cut into 4 sections.