Place each chicken breast between two sheets of heavy plastic (resealable freezer bags work well) on a solid, level surface. Firmly pound the chicken with the smooth side of a meat mallet to a thickness of 1cm. Sprinkle both sides of the flattened chicken breasts with salt, pepper and garlic powder.
Heat olive oil in a large frying pan with a cover over medium heat. Pan-fry the chicken breasts for about 2 minutes on each side, until the chicken is golden brown but not cooked all the way through. Remove the chicken and set aside.
Place the onion and garlic in the pan, and cook and stir for about 3 minutes, until the onion starts to turn translucent. Pour the white wine and chicken stock into the pan, and bring to a simmer while scraping the browned bits of food off of the bottom of the pan with a wooden spoon. Whisk in the rosemary and thyme leaves, lemon juice, lemon zest, milk and soured cream until the sauce is well mixed.
Place the chicken breasts back into the pan; cover with the sauce, Cover the pan, and simmer for about 1 hour, until the sauce is thickened. Serve chicken with sauce spooned over the top.