Place the bread crumbs in a large bowl. Cover with milk and soak for 2 minutes. Add the onion, the beaten egg, ground beef, lime juice, ketchup, garlic powder, salt, and paprika and work the ingredients together with your hands or a wooden spoon just until combined. Form the mixture into 12 small 1-inch thick patties and refrigerate them for at least 45 minutes or up to 8 hours.

Heat the olive oil in a large skillet over medium-high heat. Add about 1/2 of the burgers and brown them on each side, flipping with spatula, 5 to 7 minutes. Set the fritas on the rolls and cook the remaining burgers. Serve with generous amounts of ketchup.