Pat chicken breasts dry and pound to about 1/2-inch thickness; season both sides with garlic powder, onion powder, 1 teaspoon chopped parsley, 1/2 teaspoon thyme, 1/2 teaspoon rosemary, 1/4 teaspoon salt, and 1/4 teaspoon pepper.
Heat 1 tablespoon oil in a large skillet over medium-high heat. Add the chicken and cook about 5 minutes per side, or until opaque and cooked through. Transfer chicken to a plate and set aside.
Add 2 teaspoons oil to the same skillet and reduce heat to medium. Sauté minced garlic with the remaining 1 teaspoon chopped parsley, 1/2 teaspoon thyme, and 1/2 teaspoon rosemary for about 1 minute until fragrant.
Pour in the milk or half-and-half and season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Bring the liquid to a gentle boil.
Stir the cornstarch slurry (1 teaspoon cornstarch mixed with 1 tablespoon water) and add it to the center of the pan, whisking quickly until the sauce thickens slightly. Reduce heat and simmer for 1 minute more to thicken.
Return the cooked chicken to the skillet to warm in the sauce, spooning sauce over the pieces. Sprinkle extra chopped parsley if desired and serve immediately.
