Preheat your oven to 350°F and grease a 9×13 inch baking dish. In a large mixing bowl, combine ground beef, breadcrumbs, egg, ketchup, diced onion, minced garlic, salt, pepper, garlic powder, and Worcestershire sauce. Mix gently until just combined—don't overmix. Press the meatloaf mixture evenly into the bottom of the prepared baking dish, spreading it to the edges to create an even layer about 1 inch thick.

Bring a large pot of salted water to a rolling boil over high heat. Add elbow macaroni to the boiling water and cook for only 5-6 minutes—it should still have a firm bite and be quite undercooked. Drain the pasta but do not rinse it. The starch on the pasta helps the cheese sauce stick better.

In a medium saucepan, melt butter over medium heat. Add flour to the melted butter and whisk constantly for about 1 minute to create a roux. Slowly pour in the milk while whisking constantly to prevent lumps from forming. Keep stirring the sauce over medium heat for 3-4 minutes until it thickens enough to coat the back of a spoon. Remove from heat and stir in the shredded cheddar cheese until completely melted and smooth. Season with salt and pepper to taste. Add the drained undercooked macaroni to the cheese sauce and stir gently until every piece of pasta is coated.

Spread the mac and cheese mixture evenly over the meatloaf layer in the baking dish, spreading it all the way to the edges. Sprinkle the remaining cup of shredded cheddar cheese over the top of the mac and cheese. If using, add panko breadcrumbs on top for extra crunch. Place the baking dish in the preheated oven and bake uncovered for 40-45 minutes, until the meatloaf reaches an internal temperature of 160°F and the cheese on top is golden and bubbly. If you want an extra golden top, turn on the broiler for the last 2-3 minutes of baking, watching carefully so it doesn't burn.