Sauté the diced onion in oil until translucent. Add the ground beef to the pan and stir until no longer pink, breaking up clumps with a fork. The mixture should be crumbly.
Add the ketchup and brown sugar, and season with salt and pepper. Set aside.
Preheat 3/4 cup oil in a small saucepan until hot but not smoking.
Place eggs in a large bowl and beat well. Grate the potatoes and onion in the food processor and add to the beaten eggs. Add the salt and baking powder and mix well or until the eggs are fully incorporated into the potato mixture. Carefully pour the hot oil into the bowl. Mix very well until fully combined.
Pour a layer of oil, about 1/4 inch high into a large frying pan. Place over medium flame. Once the oil is hot, place a large heaping spoonful of potato batter into the pan. Quickly spread about 1 tablespoon of the meat filling on top of the latke, leaving about 1/2-inch border around the edge. Add another heaping spoonful of potato batter and flatten with the back of a spoon. Make sure to completely cover the meat mixture. Once the bottom of the latke is a golden brown, carefully flip and fry on the other side until done. Repeat with remaining batter.