In a small nonstick skillet, saute onion in oil until tender; set aside.
Place biscuit mix in a large bowl. In a small bowl, whisk egg and milk. Stir into the biscuit mix just until moistened. Stir in the onion mixture, 1/2 cup cheese and poppy seeds.
Transfer to an 8-in. x 4-in. loaf pan coated with cooking spray.
Sprinkle with the remaining cheese.
Drizzle with butter.
Bake at 400° for 20-25 minutes or until a toothpick inserted near the center comes out clean and loaf is golden brown. Cool for 10 minutes before removing from pan to a wire rack to cool completely. Store in the refrigerator.