Help with a mushroom recipe

Hello!

I tried making these mushroom turnovers today, but they're missing something. I'm making them again for a party Friday night, so I need some help with improving them. I don't know if making a creamy sauce would help, but they definitely need something. Help please!!


Mushroom Turnovers
2 tablespoons olive oil
1 large onion, chopped
4 packages (5 ounces each) mixed mushrooms, trimmed and thinly sliced
Coarse salt and ground pepper
1 sheet frozen puff pastry (from a 17.3-ounce box), thawed

It says to serve them with sour cream, which probably would be delicious, but they don't sell vegan sour cream out here, and I don't have the time to make some.

Thanks!