In a small bowl, mix together the apple and brown sugar. Set aside.
Pierce the squash several times with a sharp knife to let the steam escape during cooking.
Microwave on high until tender, about 5 minutes. Turn the squash after 3 minutes to ensure even cooking.
Place the squash on a cutting board and cut in half. Scrape the seeds out of the center of each half and discard the seeds.
Fill the hollowed squash with the apple mixture.
Return the squash to the microwave and cook until the apples are softened, about 2 minutes.
Transfer the squash to a serving dish. Top each half with 1 teaspoon margarine and serve immediately.
Acorn squash is a good source of vitamins A and C, potassium, and fiber. Here it's paired with apples and brown sugar to make a hearty dish. Serve along with whole-grain crackers and a small wedge of your favorite cheese to round out all food groups.