1Heat oven to 400°F. Spray 18 miniature muffin cups with cooking spray. In 7-inch skillet, melt 1 tablespoon butter over medium-high heat. Cook onion in butter 2 minutes, stirring frequently, until tender. Remove from heat; set aside.
2In large bowl, stir Bisquick mix and 1/2 cup of the cheese. In small bowl, stir egg, milk and onion with fork or whisk until well blended. Make well in center of Bisquick mixture; stir in egg mixture just until dry ingredients are moistened.
3Divide batter among muffin cups, filling each two-thirds full. Sprinkle evenly with remaining 1/2 cup cheese and the sesame seed; drizzle with 2 tablespoons butter.
4Bake 12 to 14 minutes or until golden. Serve warm.