Place bread in stacks of two. Using a serrated knife, cut each stack into 2 (2x1¼-inch) rectangles. Discard scraps or reserve for another use. To prevent from drying out, cover bread rectangles with damp paper towels. Let stand until slightly thawed, about 10 minutes.

In a small bowl, stir together mayonnaise and pesto until well combined. Spread about 1 teaspoon mayonnaise mixture over each bread rectangle.

Divide and arrange arugula over mayonnaise mixture on half of the bread rectangles. Neatly fold prosciutto halves as needed to fit bread, and place 1 over each arugula-topped rectangle. Top each with 1 slice Brie and 2 apple slices. Cover with remaining bread rectangles, mayonnaise mixture side down. Trim arugula stems and apples to neatly fit on bread slices, if needed. Serve immediately.