In a small skillet, saute onion in oil until golden brown, about 7 minutes. Set aside to cool. In a large bowl, combine buttermilk and egg. Stir in the biscuit mix, onion and 1/2 cup cheese.
Pour into a greased 9-in. round baking pan.
Sprinkle with poppy seeds and remaining cheese.
Drizzle with butter.
Bake at 400° for 30-35 minutes or until golden brown. Cool slightly.