Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 7x11-inch baking dish.

Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and crumble.

Beat eggs, milk, butter, salt, and ground pepper in a bowl; pour into prepared baking dish. Sprinkle with onions, bacon, and Cheddar cheese.

Bake in preheated oven until a knife inserted near the center comes out clean, 25 to 30 minutes.