Prick potatoes with fork. Microwave potatoes until tender.
Meanwhile, coat large nonstick skillet with cooking spray; heat over medium-high heat. Add steaks; cook, turning once, until browned, 2 minutes. Transfer to cutting board; cut into 1/4" wide slices. Add onion to skillet; cook, over medium heat, stirring, until softened and lightly browned, 7 minutes.
Horizontally cut off top 1/4 of each potato. Scoop out potato from skins into bowl, leaving 1/4" thick shell. Mash potato with milk, dip, salt and pepper. Stir in steaks, onions and 1/2 cup cheese. Spoon into potato shells; sprinkle with remaining 1/2 cup cheese. Place on jelly roll pan. Bake until hot and cheese is melted, 20 minutes.