Preheat oven to 350 degrees. Line a baking sheet with foil for easy cleanup.
In a stand mixer fitted with the paddle attachment, or in a large bowl by hand, combine beef, onion, eggs, oats, 1/2 cup ketchup, Worcestershire sauce, 1 teaspoon salt, and 1/2 teaspoon pepper. Mix on low until combined.
Rinse a 9-inch by 5-inch loaf pan with water (to help the meatloaf release). Pack the meat mixture tightly in to the pan.
Leaving a 1/2-inch wall on each side, carve out a well about 5 inches long and 1 inch wide and set the excess meat mixture aside.
Fill the well with shredded Havarti cheese. Using the reserved meat mixture, cover and seal the shredded cheese carefully.
Flip the pan over the prepared baking sheet and jiggle carefully until the meatloaf slides out. Bake 60 minutes.
Remove from oven. Increase oven temperature to 400 degrees. Brush remaining 1/4 cup ketchup on all sides of the meatloaf. Return to oven and bake 10 minutes longer.
Remove from oven and cool 15 minutes before serving.