Clean mushrooms with a damp paper towel or wash them and then dry. Remove the stems.
In 1/2 of the melted butter, sauté the onion until translucent. Cool and add in the chopped pepperoni, cheese, Italian seasoning, garlic salt and the breadcrumbs. Mix thoroughly. Use the other 1/2 of the melted butter to rub over mushroom caps. (I usually just pour it over the mushrooms and kind of gently toss them so they get coated.)
Note: As you can see in the picture, I tried to bake the mushrooms on the tray I was going to serve them from. The mushrooms cooked out a lot of liquid which I had to remove with paper towels. I never cook them this way. Normally I cook on tray with an edge to catch the juices and when they are done transfer to serving platter. I think cooking them so close together like this make them steam out more liquid than normal.