Preheat oven to 350°F. Spray a large casserole dish with cooking spray. Arrange 1/2 of the potatoes in casserole dish, dot with butter. Top with sliced onions. Sprinkle with thyme, garlic, salt and pepper.
Mix light cream with cornstarch, pour mix over potatoes evenly. Add half the cheese. Top with remaining potatoes. Sprinkle with remaining cheese.
Bake for about 55 min or until potatoes are tender and golden brown. Sprinkle with parsley, serve. If the casserole is browning too quick, cover with foil until the last 10 minutes.