In a big huge 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 gentle golden brown, 20 to 25 minutes, adjusting the warmth as wanted to stop the onions from burning (see NOTES). Add the mushrooms, season evenly 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.
