In the same skillet, melt remaining butter. Add mushrooms and cook 10 minutes. Remove from heat.
Combine eggs and half-and-half and beat until well blended.
Sprinkle about half the cheese on the bottom of tart shell. Spoon in onions. Pour in egg mixture. Place mushrooms all over the top, along with remaining cheese.
Bake 40 minutes, or until tart is golden brown and filling is set. Serve warm or room temperature.