Having the onion finely chopped is important in this recipe. It just blends better with the other ingredients.
When I chop artichokes they always seem to have one top layer that is tough. I peel that off and throw it away. The top part of the leaf is the heart so I keep that. Then chop up small and set aside.
The mushrooms will shrink when you saute them but it is still good to slice the mushrooms and then chop also.
Heat the oil in a skillet to med heat. Add the onions and saute until translucent - about 10 minutes. Remove the onions and set aside.
In the same skillet add the mushrooms and saute for about 10-15 minutes.
Remove from the pan and set aside. If there is liquid in the pan - throw that away.
In a large bowl add the onions, mushrooms, artichokes, black olives and salt and pepper.
Pour over the Italian Dressing and toss well.
This is the point where you can freeze the filling - if you are preparing way in advance. The filling will also keep very well, for at least 3 days, in the refrigerator.