In a large pot with a lid, combine the clams and 3 cups water. Cover, bring to a boil, and steam until the clams open, 6 to 8 minutes. Using tongs, transfer the clams to a large bowl, discarding any that failed to open. Boil the liquid, uncovered, until reduced slightly, about 5 minutes. Strain the clam cooking liquid through a fine-mesh sieve lined with 2 paper towels into a bowl and reserve.

In a large, clean pot, add the butter and oil and warm over medium heat. Add the garlic, leek, and celery. Sauté until the vegetables are soft and fragrant, 5 to 6 minutes. Add the wine and simmer for 1 minute. Stir in the potatoes, saffron, reserved clam cooking liquid, and clam juice and simmer until the potatoes are soft but not falling apart when pierced with a fork, 15 to 20 minutes. Add the cream and return to a simmer. Add the cod, gently shaking the pot to submerge the fish chunks, and simmer until the cod is just cooked and opaque, 5 to 7 minutes. Season to taste with salt and pepper.

Fill 4 soup bowls with a few clams and some soup, top with dill and a few grinds of pepper, and serve.