Heat oven to 400° F. In a large bowl, combine the beef, stuffing, egg, 3/4 teaspoon salt, and 1/2 teaspoon pepper. Transfer the mixture to a rimmed baking sheet and form it into a 9-inch loaf (about 2 1/2 inches thick). Bake for 30 minutes.
In a small bowl, combine the marmalade and mustard. Spread on the meat loaf and continue to bake until cooked through, 15 to 20 minutes. Let rest for 10 minutes before slicing.
Divide the arugula among plates. Drizzle with the oil and vinegar. Serve with the meat loaf.
Tip: For a tender, juicy meat loaf, choose ground chuck (20 percent fat is ideal; it’s often labeled "80/20"). Ground sirloin and other lean cuts of beef tend to dry out.