Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius). Grease a loaf pan.
In a large bowl, combine ground beef, egg, bread crumbs, chopped onion, sage leaves, garlic powder and season with salt and pepper. Mix until just combined without overworking the meat (which can make it tough).
Transfer mixture into prepared loaf pan.
Bake for 50-60 minutes or until internal temperature reaches 160 degrees Fahrenheit, checking at about halfway point to ensure even cooking.