Combine herbs and spices in a small bowl; set aside.
Heat 2 tablespoons oil or butter in large frying pan over medium heat. Saute onion until tender and golden; transfer to large bowl and set aside.
Raise heat to medium high and add 2 tablespoons oil or butter and 1/3 of shredded mushrooms to pan, stirring to coat. Add 1/3 of spice mixture and stir occasionally, at least 5 minutes. Deglaze with a splash of vinegar and transfer to bowl with onions.
Repeat step 3 in two more batches.
Return mushrooms and onions to pan; add water or broth and BBQ sauce. Stir to combine and simmer until thickened.
To serve, toast or warm buns and top with condiments of your choice.