Wash potatoes; pierce with fork. Arrange potatoes on microwave-safe plate in spoke formation, leaving 1 inch between each potato. Micro-cook on high (100 percent power), uncovered, 14 to 18 minutes or until tender. (Or bake potatoes in 425 degree F oven for 40 to 60 minutes.) Let stand 5 minutes.
Meanwhile, preheat broiler. Trim fat from steak. Place meat on unheated rack of broiler pan. Broil 3 to 4 inches from heat for 16 to 18 minutes for medium-rare (145 degrees F) or 19 to 21 minutes for medium (160 degrees F), turning once halfway through broiling. Transfer meat to cutting board; let stand 5 minutes. Or, to grill, prepare steak as above. For charcoal grill, grill on rack of uncovered grill directly over medium coals 18 to 22 minutes for medium (160 degrees F), turning once. For gas grill, preheat grill. Reduce heat to medium. Place meat on grill rack over heat. Cover; grill as above.
To serve, roll each potato gently under your hand. Cut an X in top of potato. Press in and up on ends of potato. Cut steak in bite-size strips. Top potatoes with beef strips and spinach; drizzle with dressing. Top with onion wedges. Makes 4 servings.