unsalted butter • large yellow or red onion, finely chopped • Kosher salt and black pepper • celery stalks (ribs), chopped • garlic cloves, smashed and chopped • white wine • chicken stock • boneless, skinless chicken thighs, cut in 3 to 5 pieces each • lemon, zested and juiced • Parmesan cheese rind (optional) • red-pepper flakes • frozen cut spinach •