Using a large, deep skillet, brown your sausage over medium heat. If using sausage meat, break up with the back of a spoon into crumbles, then remove from pan and put aside. If using links, brown evenly and also set aside until cool enough to slice.
Remove any extra grease from the pan other than what is needed to saute the onions, if necessary. Add them to the pan over a low-medium heat and soften slightly before adding the zucchini slices.
Saute for a few minutes before adding the mushrooms.