Grate 1/2 the apple into a large bowl. Slice the remaining half into 8 thin wedges and set aside.
Add yogurt, thyme, rosemary, salt, pepper and garlic salt to grated apple. Lay the chicken breasts onto the counter on a piece of plastic wrap, and cover with a second sheet of plastic wrap.
Pound the thick end of the chicken with your fists to even out thickness. Remove the plastic wrap, spread a heaping tablespoon of yogurt mixture into the center of each chicken breast, and lay apple slices in the center. Roll chicken and lay into a casserole dish that has been oiled slightly. Secure each with a toothpick if necessary.
Smear any remaining yogurt onto breasts. Cover with foil and bake at350°F (180°C) for 20 minutes, remove foil and bake for 20 more minutes or until chicken is cooked through.
Slice apple chicken roll-ups crosswise and serve hot, or chill in fridge and serve cold with salad greens.