1Heat oven to 425°F. Spray 12 regular-size muffin cups with cooking spray.
2In 8-inch skillet, melt butter over medium-high heat. Cook onion in butter 3 to 5 minutes, stirring frequently, until tender. Stir in salmon.
3In large bowl, stir Bisquick mix, 3/4 cup of the cheese, the dill and lemon peel. In small bowl, stir milk, oil and egg with fork or whisk until blended. Make well in center of Bisquick mixture; stir in egg mixture just until dry ingredients are moistened. Stir in onion and salmon. Divide batter evenly among muffin cups, filling each two-thirds full. Sprinkle evenly with remaining 1/2 cup cheese.
4Bake 18 to 20 minutes or until golden. Cool 2 to 3 minutes; remove from pan. Serve warm.