Soak the clams in a large bowl of cold water, and sort through them, discarding any shells that remain open or are cracked or smashed.
Heat the oil in a large, heavy saucepan that comes with a lid, then add the chopped shallot, stirring for a minute, grate in (or mince and add) the garlic, and add the red pepper flakes, stirring again over the heat so that it sizzles, though not long enough to let the garlic brown.
Stir in the tomato paste, then add the broth and the vermouth and let it come to a boil.
Add the fregola (it should be covered completely by the liquid) and let it simmer, still uncovered, for 10-12 minutes (or as instructed on the fregola package).
Check that the fregola is nearly ready and then add the drained clams, and cover the pan with the lid. Let cook for 3 minutes at a fast simmer, then uncover the pan to check that the clams have opened. Any clams that, once cooked, stay closed should be discarded.
Sprinkle in the chopped parsley and stir to let everything combine before ladling into 4 warmed bowls to serve, sprinkling with a little more chopped parsley as you go.
Reprinted from Nigellissima. Copyright (c) 2012 by Nigella Lawson. Published by Clarkson Potter, a division of Random House LLC. All rights reserved.