Combine first 7 ingredients in a small bowl, stirring well.
Trim fat from steak; slice steak diagonally across grain into 1/4-inch-wide strips. Add steak to onion mixture. Cover and marinate in refrigerator 2 hours.
Scrub potatoes. Bake at 400° for 35 minutes or until tender. Let cool to touch. Cut potatoes in half lengthwise. Carefully scoop out pulp, leaving 1/4-inch-thick shells; set shells aside. Mash pulp.
Coat a small nonstick skillet with cooking spray; place over medium heat until hot. Add meat mixture; cook 2 minutes or until meat is browned on both sides. Add mashed potato to skillet, and cook 1 minute.
Place potato shells in a small baking dish. Spoon steak mixture evenly into shells, and top with cheese. Bake at 350° for 10 to 12 minutes or until thoroughly heated. Top evenly with salsa.