Preheat oven to 450°.
Rinse potatoes; pierce several times with a fork. Bake at 450° for 45 minutes or until tender.
Heat oil in a large nonstick skillet over medium-high heat; add onions and garlic. Cook 1 minute or until soft. Add spinach; cook 2 minutes or until thoroughly heated. Remove from pan, and place in a large bowl.
When potatoes are cool enough to handle, cut in half lengthwise, and scoop out pulp, leaving a 1/4-inchthick shell. Add pulp to spinach mixture; stir in 1/4 cup Parmesan cheese and next 4 ingredients. Place potato shells in an 11 x 7-inch baking dish coated with cooking spray. Spoon filling evenly into potato shells. Bake 15 minutes or until thoroughly heated and lightly browned. Top with remaining 1/4 cup Parmesan cheese and, if desired, salsa.