Preheat oven to 325 degrees F. Coat a 1-quart baking dish with fat-free cooking spray.
In a medium saucepan over medium heat, melt the margarine. Add the onion and cook until tender (about 5 minutes).
Whisk together the half-and-half and cornstarch until the cornstarch dissolves. Pour into the pan and cook, whisking frequently, until the mixture thickens, 2 to 3 minutes. Remove from the heat and stir in the corn, egg, sugar substitute, salt, and pepper. Pour into the prepared baking dish and sprinkle with the paprika.
Bake until bubbly and set on the surface, 55 to 60 minutes.