Preheat oven to 325 degrees.
Spray a pyrex pan.
Spread the separated onions in the bottom of the pan
Lay downt he tilapia in a single layer so that it fills up most of the pan.
Pour the mushroom pieces and the condensed cream of mushroom soup over the fish; spread evenly.
Top with sprinkled breadcrumbs.
Baked 30 or so minutes until breadcrumbs are browned a little.