Preheat the oven to 375 degrees and lightly grease a 9x9 baking dish.
Cook the macaroni according to package directions, adding the frozen peas during the last two minutes. Drain and set aside.
Cook the bacon in a skillet until crisp, then drain on paper towels and crumble into small pieces.
In a large bowl, mix the ground beef, chopped onion, egg, breadcrumbs, garlic powder, salt, and pepper until evenly combined.
Stir in half of the shredded cheddar and half of the crumbled bacon, then fold in the cooked macaroni and peas.
Spread the mixture into the prepared baking dish and top with the remaining cheddar and bacon.
Bake for 35 to 40 minutes until the top is golden and the center reaches 160 degrees. Let it rest for five minutes before serving.