Melt 1 stick of butter in a medium skillet over medium heat.
Dice 1 medium onion and add it to the skillet.
Cook until the onion is soft and translucent, about 5-7 minutes. Remove from heat and let cool slightly.
In a large mixing bowl, combine 1.5 lbs ground hamburger meat, 1 egg, 2 tablespoons Worcestershire sauce, 1/2 cup milk, 1/3 cup ketchup, and 2/3 cup old fashioned oats.
Add salt and pepper to taste.
Mix thoroughly using your hands or a large spoon.
Add the cooled, cooked onions to the meat mixture.
Mix well to ensure the onions are evenly distributed.
Preheat the oven to 350°F (175°C).
Transfer the meat mixture to a loaf pan or shape it into a loaf in a casserole dish.
Spread a generous layer of ketchup over the top of the meatloaf.
Bake for approximately 1 hour, or until an internal temperature of 165°F is reached.
Let the meatloaf rest for a few minutes before slicing.
Serve hot with your favorite sides.