Preheat oven to 350 °F and grease a baking dish.
Cook ground beef and chopped onion in a large skillet over medium heat until browned.
Add minced garlic, salt, pepper, onion powder, mustard, and ketchup; simmer briefly and remove from heat.
Boil salted water in a pot, cook pasta until just al dente, then drain.
Whisk together milk and eggs in a large bowl until smooth.
Combine cooked pasta and beef mixture with milk-egg mixture, stirring well.
Pour mixture into prepared baking dish and sprinkle shredded cheese evenly on top.
Bake for 25 to 30 minutes until cheese is melted, bubbly, and lightly golden.
Let cool slightly before serving, adding optional toppings if desired.