Heat olive oil in a wide pan and sauté garlic, parsley, and chilli flakes for one minute.
Add the fish stock and half of the chopped tomatoes to the pan and bring to a boil.
Simmer the fregola pasta in the broth for ten minutes until just cooked.
Cover and steam clams in a separate pan with 5mm of boiling water for 3 to 5 minutes.
Remove opened clams with a slotted spoon and discard any that remain closed.
Strain the clam cooking liquor into a bowl to remove any sediment.
Stir the clams, strained liquor, saffron water, lemon zest, and remaining tomatoes into the pasta.
Ladle the soup into bowls, drizzle with olive oil, and serve immediately.