Toss spinach with mushrooms, cheese and onions in large bowl; set aside.
Cook bacon in large skillet on medium heat to desired crispness, turning frequently.
Remove bacon from skillet; reserve 2 Tbsp. of the drippings in skillet.
Place bacon on paper towels to drain.
Add walnuts to reserved drippings in skillet. Cook 1 to 2 min. or until lightly toasted, stirring occasionally. Stir in vinegar, oil, salt and pepper. Cook 30 sec. or until heated through.
Pour hot dressing over salad; toss to coat. Top with eggs. Crumble bacon; sprinkle over salad.