To make the marinade, combine the olive oil, balsamic vinegar, garlic, Italian seasoning, salt, and pepper in a dish and stir until combined. Place the chicken breasts in a zip-top gallon bag and pour the marinade over the top. Seal the bag, removing any excess air, and toss/massage the contents until the chicken breasts are coated in marinade. Place the bag in the fridge and allow the chicken to marinate for at least one hour/up to 12 hours.
Just before you’re ready to grill the chicken, make the topping by combining the diced strawberries, brown sugar, balsamic vinegar, black pepper, and basil and stirring together. Allow to sit for 10-15 minutes.
Heat your grill to medium. Remove the breasts from the marinade and allow any excess to drip off into the bag. Discard the leftover marinade. Place the breasts on the grill and cook, flipping halfway through, until the breasts reach an internal temperature of 165˚F. Remove from heat.
Top each chicken breast with ¼ cup of the strawberry topping before serving.