In a pot over medium heat add the onions, olive oil and butter, cook stirring occasionally until onions have caramelized, about 20 minutes (do not let the heat get too high or the onions will burn before they caramelize).
Add both kinds of mushrooms and sage, cooking until the mushrooms have softened and darkened, about 5 minutes.
Add the stout and stock, simmer for fifteen minutes.
Add the orzo and black pepper, cooking until orzo is cooked through, about 8 minutes. Salt to taste.