Wipe mushrooms, trim the stems, and slice. Then slice in half. Set aside.
Heat olive oil in a large pan on medium heat. Add onions. Cook until translucent, about 5 minutes.
Add garlic. Cook 30 seconds.
Add sliced mushrooms. Cook about 5 minutes.
Add BBQ seasoning, salt, pepper, and BBQ sauce. Stir and cook for a couple of minutes or so.
Assemble sandwiches with BBQ mushrooms, sliced pineapple, and cilantro.
Notes
Add salt at the end of cooking. Salt brings out moisture in the mushrooms.Don't clean mushrooms until you're ready to cook them.I like to wipe them with a damp towel. Keep in mind they soak up water and can get waterlogged, so if you wash them make sure to dry them completely.The mushrooms and onions will last about 7 days in an airtight container in the fridge.