Blackened Portobello-Mushroom Salad

INGREDIENTS

• 1/4 cup red wine vinegar
• 1/4 cup balsamic vinegar
• 1/4 cup tomato juice
• 1 tablespoon olive oil
• 2 teaspoons Dijon mustard
• 2 teaspoons stone-ground mustard
• 1/4 teaspoon coarsely ground pepper
• 4 (4-ounce) portobello mushroom caps (about 5 inches wide)
• 1 tablespoon Cajun seasoning for steak (such as Chef Paul Prudhomme's Steak Magic)
• 2 teaspoons olive oil
• Cooking spray
• 16 cups gourmet salad greens
• 1 large tomato, cut into 8 wedges
• 1/2 cup thinly sliced red onion, separated into rings
• 1 (15-ounce) can cannellini or other white beans, rinsed and drained
• 1/4 cup (1 ounce) crumbled blue cheese