Spread the honey mustard evenly over the bread. Top with the turkey, nectarine slices, and Brie. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper.
In a large bowl, toss the arugula, onion, oil, and 1/2 teaspoon salt. Serve with the sandwiches.
Upgrade: Adding fruit is a delicious and easy way to spruce up almost any sandwich with cheese. Some more great pairings: plums and Parmesan, apples and Cheddar, pears and blue cheese, and figs and goat cheese.