Preheat oven to 400°F (200°C).

Halve the acorn squash and remove the seeds.

Spread 1/2 tablespoon of margarine on each of the 4 squash halves.

Place the squash halves, cut side down, in a 9 x 13-inch baking dish.

Bake at 400°F (200°C) for 30 minutes.

While the squash is baking, sauté the chopped onions in 1 tablespoon of margarine until tender.

In a mixing bowl, combine the cooked brown rice, sautéed onions, 3/4 cup of shredded Cheddar cheese, and minced fresh parsley.

Remove the squash from the oven and turn them cut-side up.

Fill each squash half generously with the rice mixture.

Sprinkle the remaining 1/4 cup of Cheddar cheese evenly over the stuffed squash halves.

Reduce the oven temperature to 350°F (175°C).

Bake for another 30 minutes at 350°F (175°C), or until the cheese is melted and golden brown.