Pierce potatoes with a fork and place on microwaveable plate. Microwave (HIGH) until fork tender (about 3-4 minutes). Cut potatoes in half; set aside to cool.
Place mushrooms, onions, potatoes and ½ cup vinaigrette in a 1-gallon recloseable food storage bag. Seal bag; gently toss to coat.
Prepare grill to medium heat. Place vegetables on grill; grill, turning occasionally until golden brown and mushrooms are soften (about 10 minutes). Remove from grill and allow to cool slightly; cut into bite size pieces.
Meanwhile, season steaks with pepper to taste; place steaks on grill. Grill, turning once, 7-15 minutes (depending on thickness) or until desired doneness is reached (145°F for medium-rare; 160°F for medium). Remove from grill; season with salt to taste; slice.
In large bowl, toss spinach, tomatoes, cheese and grilled vegetables with remaining salad dressing. Divide among dinner plates. Top with steak slices.