Remove 1/4 teaspoon zest and squeeze 1/4 cup juice (and, if using, 1/3 cup additional juice) from orange(s).
For sauce, stir brown sugar and cornstarch in a small saucepan. Stir in orange zest, 1/4 cup orange juice, and the next four ingredients (through pepper). Cook and stir over medium-high heat until thickened and bubbly. Cook and stir 1 minute more.
Meanwhile, warm broth in a large skillet over medium heat. Add turkey; cook, covered, 5 minutes or until heated through. Arrange mashed potatoes, arugula, turkey, and sauce on toasted bread.