Preheat the oven to 350 degrees F. In a large bowl, combine the ground beef, breadcrumbs, beaten egg, chopped onion, salt, and pepper until evenly mixed.
Shape the mixture into a 2-pound loaf and place it on a baking sheet lined with parchment paper.
Bake the meatloaf for 45 minutes.
While it bakes, combine the apple cider, brown sugar, and Dijon mustard in a small saucepan. Bring to a simmer over medium heat and cook, stirring occasionally, until the mixture reduces by about half and thickens slightly.
Remove the meatloaf from the oven, brush the glaze evenly over the top and sides, then return it to the oven for 15 more minutes until the glaze sets and the internal temperature reaches 160 degrees F.
Let the meatloaf rest for 10 minutes before slicing.