Melt butter in a skillet over medium-high heat; add onion, and sauté 3 to 5 minutes or until tender.
Combine baking mix and half of cheese in a large bowl; make a well in center of mixture.
Stir together milk, Dijon mustard, and egg, blending well; add to cheese mixture, stirring until moistened. Stir in onion and ham. Spoon into lightly greased muffin pans, filling two-thirds full. Sprinkle with remaining 1 cup cheese. Sprinkle with poppy seeds, if desired.
Bake at 425º for 18 minutes or until golden. Let stand 2 to 3 minutes before removing from pans.
Note: Substitute mini muffin pans for regular pans, if desired. Bake at 425º for 14 minutes or until golden. Makes 2 1/2 dozen mini muffins.