Is there a reason you don't want to use sausage? My son and DDiL are vegan but don't mind meat analogs so soysage makes a nice appetizer for all.
You don't have to, you can just chop up the stems, chop up some onion, saute together and then season as you wish, I usually go towards Asian flavors but spinach and parmesan maybe with a dash of balsamic vinegar or worcestershire works nicely too. Pine nuts can add some texture. Stuff back into the caps and away you go.