Melt butter in a skillet over medium-high heat; add onion, and saut 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 425 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 425 for 14 minutes or until golden. Makes 2 1/2 dozen mini muffins.