How to Make It
In a large pot, bring the Fish Stock to a simmer. Add the clams, cover and cook over moderately high heat until nearly all have opened, about 8 minutes. Add the mussels and cook until opened, 2 to 3 minutes. Using a slotted spoon, transfer the clams and mussels to a bowl; remove them from their shells. Strain the stock into a heatproof measuring cup. Wipe out the pot and return 3 cups of the fish stock to the saucepan, stopping before you reach the grit at the bottom. Stir in the saffron and let stand for 5 minutes.
In a medium saucepan, combine the ditalini with 1/2 cup of the stock and bring to a simmer. Cook over moderate heat, stirring constantly, until the stock is absorbed. Continue adding stock 1/2 cup at a time, allowing it to be absorbed before adding more. The ditalini is done when it is al dente and suspended in a thick, rich sauce, about 25 minutes. Stir in the garlic, mussels and clams and cook 1 minute longer. Transfer the pasta to bowls. Sprinkle with the parsley, drizzle with olive oil and serve.