In a small skillet, saute onion in butter until tender. In a large bowl, combine the onion, eggs, milk, oats, parsley and seasonings. Add beef and mix well. Press into an 8x4-in. loaf pan lined with waxed paper. Refrigerate for 2 hours.
Unmold loaf into a shallow baking pan. Bake at 350° for 30 minutes. Meanwhile, combine glaze ingredients. Remove loaf from oven; drain. Brush with some of the glaze. Return to oven; bake 1 hour longer or until no pink remains, occasionally brushing with glaze.