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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.
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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.
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3
Slice chicken breasts; lay Brie cheese on top to melt.
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4
Preheat oven to 300 degrees F (150 degrees C).
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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.
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6
Transfer skillet to the preheated oven; bake tomatoes until softened, about 15 minutes.
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7
Combine remaining 2 teaspoons olive oil, 1 tablespoon balsamic vinegar, and 1/8 teaspoon salt in a bowl. Add spinach; toss to coat.
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8
Mix mayonnaise, Dijon mustard, garlic, and pepper together in a bowl.
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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.
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10
Bake in the preheated oven until heated through, 5 to 10 minutes. Cut into 4 sections.
