Melt 1 tablespoon butter in a skillet over medium-high heat; add onion, and sauté until tender.
Combine onion, biscuit mix, and 1/2 cup cheese in a medium bowl. Stir in egg and milk just until moistened. Spoon into greased muffin pans, filling half full. Sprinkle with remaining 1/2 cup cheese and sesame seeds; dot with remaining 1 tablespoon butter.
Bake at 400° for 12 minutes or until golden. Remove from pans immediately, and serve warm.