Preheat oven to 325 degrees. Thinly slice onions and lightly brown in tablespoon of butter. Set aside. Whisk eggs and light cream together. Add ham, mushrooms, caramelized onions, cheese, thyme leaves, salt and pepper. Whisk together until combined. Prick pie crust a few times with fork. Pour mixture into pie crust. Bake for 1 hour or until set.