8. (Optional) For deeper flavor, heat the oil and butter in a skillet first and sauté the onion 3–5 minutes, then add the shallots, garlic, celery, carrots, thyme, rosemary, crushed fennel, and a pinch of salt and pepper for 3–4 minutes; transfer everything to the slow cooker. If you prefer, you may skip the sauté and add the raw aromatics directly to the slow cooker.