Grilled Blue Cheese Sandwiches

Ingredients (11)

• 1-1/2 cups sliced fresh mushrooms
• 2 tablespoons chopped onion
• 1/2 cup mayonnaise, divided
• 2 cups shredded cheddar cheese
• 1/4 cup crumbled blue cheese
• 1 teaspoon yellow or Dijon mustard
• 1 teaspoon Worcestershire sauce
• 1/4 teaspoon salt
• 1/8 teaspoon cayenne pepper
• 12 slices white or wheat bread
• 2 to 4 tablespoons butter