Roasted Cauliflower Grilled Cheese

small head of cauliflower, sliced into flat, 3/8-inch-thick pieces

small yellow onion, sliced into 1/4-inch-thick rounds

olive oil

Salt and freshly ground black pepper,

butter, softened

slices whole-grain bread (such as Eureka! Organic Bread)

slices Arla Dofino® Gouda cheese (or 2 ounces, grated)

slices Arla Dofino® Creamy Havarti cheese (or 2 ounces, grated)

baby arugula

light mayonnaise

small garlic clove, minced

whole grain mustard

fresh lemon juice

Salt and freshly ground black pepper,