Preheat oven to 375°F (190°C) and grease a 9x13-inch baking dish.
Heat olive oil in a large skillet over medium-high heat. Add diced onion and ground beef. Cook until the meat is browned and onions are softened, about 7-8 minutes. Drain excess fat.
Spread the cooked meat and onion mixture evenly across the bottom of the prepared baking dish. Sprinkle 1 cup of shredded cheddar cheese on top.
In a large mixing bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and fresh parsley until well combined.
Pour the egg mixture over the meat and cheese in the baking dish. Top with the remaining 1/2 cup of cheddar cheese.
Bake uncovered for 25-30 minutes, or until the eggs are set in the center and the edges are lightly golden.
Let cool for 5 minutes before slicing and serving.