Heat the oven to 350°. On the stove top, melt the butter in a 10-inch cast-iron or other oven-safe skillet (if you don't have one, pour melted butter into a 10-inch deep-dish pie pan or round casserole). Add the fruit and lemon juice to the skillet and stir over low heat for 1 minute. Remove the pan from the heat. In a small bowl, combine the sugar, cornstarch, and cinnamon, then stir into the fruit. Smooth the fruit with a spoon.