Bring a large pot of water to a boil, and immerse the zucchini in the boiling water. Set heat to medium, and cook the zucchini until they feel slightly soft when pressed with a wooden spoon, about 20 minutes.
Drain the zucchini, and set aside to cool.
Heat the vegetable oil in a skillet over medium heat, and cook and stir the onion and garlic until the onion is translucent, about 5 minutes. Stir in the ground beef, and cook and stir the meat, breaking it up as it cooks, until the beef is no longer pink, about 8 minutes. Stir in the paprika, and set the beef mixture aside.
Cut the zucchinis in half lengthwise, and scoop out the flesh with a spoon, leaving a 1/2-inch thick shell. Chop the scooped-out zucchini flesh, and mix into the ground beef mixture.
Preheat oven to 350 degrees F (175 degrees C).
In a saucepan over low heat, melt the butter and whisk in the flour until smooth.
Pour in the milk, a little at a time, and bring the mixture to a simmer, whisking constantly.
Whisk in white pepper, nutmeg, and salt, and stir in half the Parmesan cheese to create a smooth sauce.
Mix the white sauce thoroughly with the ground beef mixture.
Place the zucchini halves into a baking dish. Fill the zucchini shells with the creamy beef mixture, and sprinkle each with about 1 1/2 tablespoons of the remaining Parmesan cheese.
Bake in the preheated oven until the Parmesan cheese topping begins to turn golden brown, about 15 minutes.
