Heat olive oil in a skillet over medium-high heat. Place chicken and garlic in the pan. Season with 1/2 teaspoon of salt, pepper, and Italian seasoning. Stir in the tomatoes, olives, wine, and parsley. Reduce heat to low and continue cooking until the chicken is no longer pink in the center. Remove chicken and place on a plate, leaving the sauce in the pan. Stir sour cream, milk, and 1/2 teaspoon of salt into the remaining sauce.
Whisk cornstarch and water together in a small bowl. Increase heat to medium and whisk in the cornstarch mixture. Continue stirring until the sauce has thickened. Serve the sauce with the chicken.