Prepare the romesco sauce: If using a dried pepper, remove the stem and seeds from the pepper and soak in warm water for 1 hour, or until softened. Scrape off the flesh and discard skin. If using a roasted pepper, remove the charred skin, stem, and seeds, and chop. Set aside.
In a food processor, finely chop the bread and almonds. Add the salt, pepper, garlic, tomato and reserved flesh of the pepper. Process to a smooth paste. With the motor running, add the olive oil in a thin stream, until mixture is the consistency of a thick, creamy sauce. Add the warm water as needed to help thin the sauce if necessary. Adjust the seasonings, transfer to a bowl and set aside at room temperature or chill until ready to serve.
Preheat a grill or broiler, and preheat an oven to 400 degrees F. Trim the roots off the onions and spread them over the grill. Grill the onions until charred all over but not dried out, turning frequently. Transfer the onions to a large roasting pan, cover tightly and roast in the oven for 20 to 25 minutes. Serve the onions steaming hot, strip back the charred outer skin, dip the onions in the romesco sauce, eat the white bottoms and discard the top.
c.1997, M.S. Milliken & S. Feniger, all rights reserved