In a big vast saucepan or skillet over medium warmth, soften the remaining 1/2 cup (4 ounces) of schmaltz. Add the onions and cook dinner, stirring often, till mild golden brown, 20 to 25 minutes, adjusting the warmth as wanted to forestall the onions from burning (see NOTES). Add the mushrooms, season frivolously with the remaining salt and pepper and cook dinner, stirring, till the mushrooms give off most of their liquid and the onions flip darker, about 10 minutes. Style, and season with further salt and/or pepper, if desired. Switch the mushroom-onion combination to the pot with the pasta.
