Roasted Cauliflower Grilled Cheese

For the Sandwich:

• 1 cauliflower sliced into flat, 3/8-inch-thick pieces, small head
• 1 yellow onion sliced into 1/4-inch-thick rounds, small
• 1 tablespoon olive oil
• salt to taste, freshly ground
• black pepper to taste, freshly ground
• 2 tablespoons butter softened
• 4 lices whole-grain bread such as Eureka! Organic Bread
• 2 lices gouda cheese or 2 ounces, grated, Arla Dofino®
• 2 lices havarti cheese or 2 ounces, grated, Arla Dofino® Creamy
• 1/3 cup arugula baby

For the Lemon Mustard Aioli:

• 1/4 cup mayonnaise light
• 1 garlic minced, small clove
• 2 teaspoons whole grain mustard
• 2 teaspoons lemon juice fresh
• salt to taste, freshly ground
• black pepper to taste, freshly ground