1. To cook the sweet potatoes in a conventional oven, bake them at 400°F for about 45 minutes, or until tender. To cook in a microwave oven, prick each potato in several places with a fork, and microwave at high power for about 15 minutes, or until tender. Set aside to cool; then peel and cut into bite-sized pieces.
2. Place the potato pieces in a large bowl and toss with all of the remaining ingredients except the marshmallows. Coat a 2 1/2-quart casserole dish with nonstick cooking spray, and place the sweet potato mixture in the dish. Top with the marshmallows.
3. Bake at 350°F for 35 to 40 minutes, or until the sweet potato mixture is bubbly and the top is lightly browned. If the top starts to brown too quickly, loosely cover the dish with aluminum foil during the last 10 minutes of baking.