Heat oven to 450° F. Line shallow baking pan with foil; spray with nonstick cooking spray. Cut 3/4-inch deep x 2 ½-inch long slits down center if each salmon fillet. Place skin side down on prepared baking pan; sprinkle salmon with salt. Set aside.
Melt 1 tablespoon butter in 10-inch nonstick skillet over medium-high heat. Add onions; cook 2 minutes. Add red bell pepper and mushrooms. Cook 2-3 minutes until vegetables are soft. Add spinach; cook 1 minute until wilted.
Reduce heat to low. Slowly stir in Wisconsin 1/3 less fat cream cheese until cheese is smooth. Carefully stuff filling into cut salmon fillets (filling will mound above salmon).
Melt remaining 1 tablespoon butter. Stir together breadcrumbs, Grated Parmesan cheese and melted butter in small bowl. Carefully top each stuffed salmon with bread crumb mixture**.
Bake 12-15 minutes or until salmon is no longer pink.