Heat butter and olive oil in a large skillet on medium flame. Add the bacon or pancetta (if using) and brown. Add onions and cook for a few minutes, until translucent and soft but not browned. If using mushrooms, push the onions to the side and brown the mushrooms. Add the chicken, if desired, and just heat it through. Add the spinach and cook until just wilted. Add the spaetzle, toss to coat with oil, and cook until they get a bit crispy in spots.