How to Make It
In an enameled cast-iron casserole or Dutch oven, combine the olive oil with the onion, celery, carrot, garlic, lemon zest, saffron, fennel seeds and herbes de Provence; season with salt and pepper. Add the chicken, toss to coat and let stand at room temperature for 15 minutes.
Add the tomatoes, wine, potatoes and 3/4 cup of water to the casserole. Cover and bring to a boil over high heat. Cover partially and simmer over low heat until the potatoes are half-cooked, 25 minutes. Add the kielbasa and simmer until the chicken is cooked through and the potatoes are tender, 10 minutes longer. Stir in the tarragon; keep warm.
Transfer 2 potato chunks to a food processor along with 1/4 cup of the cooking liquid from the stew. Add the garlic, paprika and cayenne and pulse to combine. Add the mayonnaise and puree. With the machine on, add the olive oil in a thin stream and process until smooth and creamy. Season the rouille with salt.
Serve the bouillabaisse in shallow bowls and spoon the rouille on top.