For the meatloaf topping:
Preheat oven to 375°F. Grease a sheet pan with nonstick cooking spray.
In large bowl, combine the ground beef, onions, egg, oats, ketchup, dry mustard, brown sugar, sage and salt and pepper. Use a fork and then your hands to gently bring it together. Don't overmix or the meatloaf will become tough.
Form the mixture into a log about 4-5 inches thick on the sheet pan.
Whisk the ketchup, brown sugar, and dry mustard together to make the topping. Spread on top of the meatloaf.
Bake for 10 minutes.
Meanwhile, toss the potatoes with the olive oil and plenty of salt and pepper.
Spread the potatoes out onto the sheet pan around the meatloaf.
Bake for another 25-30 minutes until potatoes are tender and an instant-read thermometer inserted into the center of the meatloaf registers 165°F.
Cut meatloaf into slices and serve with potatoes.
Calories: 545kcal, Carbohydrates: 39g, Protein: 26g, Fat: 32g, Saturated Fat: 10g, Cholesterol: 121mg, Sodium: 1006mg, Potassium: 954mg, Fiber: 4g, Sugar: 17g, Vitamin A: 239IU, Vitamin C: 16mg, Calcium: 87mg, Iron: 7mg