Preheat oven to 450°.
Pierce potatoes with a fork, and bake at 450° for 1 hour or until done; cool slightly. Reduce oven to 350°.
Place a nonstick skillet over medium-high heat until hot. Coat onions and garlic with cooking spray; add to pan and sauté 5 minutes or until tender. Remove from heat.
Cut each potato in half lengthwise; scoop out pulp, leaving a 1/4-inch shell. Mash pulp with a potato masher. Add onion mixture, sour cream, salt, and pepper to pulp.
Spoon potato mixture into shells; sprinkle evenly with Parmesan cheese. Place stuffed potatoes on a baking sheet; bake at 350° for 15 minutes or until thoroughly heated.