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TOSS onions with 1/4 cup dressing in medium microwaveable bowl; cover with waxed paper. Microwave on HIGH 1-1/2 min. or until onions are crisp-tender. Add mushrooms, peppers and mozzarella; mix lightly.
BRUSH remaining dressing evenly onto cut sides of baguette; cover bottom half with provolone, spinach and onion mixture.
BROIL both halves, 6 inches from heat, 3 min. or until cheese is melted. Cover bottom half of baguette with top of baguette. Cut into pieces to serve.