Remove stems from mushrooms, and dice. Set mushroom caps aside.
Melt 1/4 cup butter in a skillet over low heat; add onion, and sauté until tender. Add diced mushroom stems, and sauté until tender. Transfer to a small bowl; stir in chopped ham and next 4 ingredients.
Melt remaining 1/4 cup butter in skillet over medium heat; add mushroom caps, and sauté until barely tender. Remove from pan.
Stuff caps with ham mixture, and place on a lightly greased baking sheet. Cover and chill up to 12 hours, if desired.
Broil 5 inches from heat (with electric oven door partially open) 6 to 8 minutes or until filling is lightly browned.