I have my heart set on a recipe of orange roughy prepared with a sauce of fennel, onion and tomato. I searched for fennel in two different grocery stores without success. What do think a good substitute would be for the fennel bulb? The recipe calls for a low and slow sauté of the onions and fennel. I have celery and napa cabbage in my frig. What if I combine that with some fennel seeds? Any other ideas? I have never cooked with fennel before.