Soak the salt cod in cold water in a large bowl for 4 hours, changing the water 3 times, and then drain, discarding the water.
Place the salt cod, onion, leeks, potatoes, escarole, garlic, olive oil, wine, thyme and water in a large stockpot, bring to a boil, reduce heat and let it simmer gently for about 1 hour. The russet potatoes should just start to thicken the stew. Add the beans and season to taste. (Can be made a day in advance, refrigerated overnight.)
When ready to serve, heat 1 tablespoon canola oil in a large sauté pan over medium-high heat. Add 4 fish fillets at a time (without crowding, so they brown properly) and sear until golden on 1 side, then remove to a plate next to stove. Repeat for next 4 filets. Return fish to skillet, seared side up, add clams, cover and heat until clams are open.
Meanwhile, double-check the broth for seasoning, adding a sprinkle of salt or a squeeze of lemon juice if necessary (and you might want to add some of the liquid from the sauté pan to the soup pot).
To serve, ladle broth in 8 warm shallow bowls and place a cod filet and 4–5 clams in each with a sprinkle of parsley or chives, a drizzle of olive oil and a wedge of lemon. (You can also offer this buffet style and let guests serve themselves.) Serve with thick slices of toasted focaccia to soak up the broth.
Recipe by Steve Johnson, The Red Dory, Tiverton