Preheat the oven to 350 degrees. In a large bowl, combine the ground beef, breadcrumbs, milk, onion, salt, and pepper. Mix until just blended, then shape into a loaf and place on a baking sheet. Spread the ketchup evenly over the top.
Bake the meatloaf for 50 minutes until it reaches an internal temperature of 160 degrees. Remove from the oven and let it rest for 5 minutes before slicing.
While the meatloaf bakes, prepare the biscuits. In a mixing bowl, whisk together the flour, baking powder, and salt. Cut in the cold butter until the mixture resembles coarse crumbs. Stir in the buttermilk until a soft dough forms.
Turn the dough onto a floured surface, pat it to 1-inch thickness, and cut out 4 biscuits. Place them on a baking sheet and bake for 12 to 15 minutes until golden.
For the gravy, melt the butter in a skillet over medium heat. Whisk in the flour and cook for 1 minute. Gradually add the milk while whisking, then season with salt and pepper. Cook until the gravy thickens, about 3 to 4 minutes.