Timing note: The flavor of the spread improves if made 1 or 2 days before serving. The spread will keep for up to 1 week.
Light a medium fire in a charcoal grill. Set the bell peppers and the chilies on the grill and cook slowly, turning occasionally, until they are blackened and blistered all over, about 15 minutes. Alternatively, in batches, set the peppers and chilies on a gas burner with the flame turned to medium-high and rotate them, as needed, until blackened and blistered all over. Transfer the peppers and chilies to a large bowl and cover with a plate or plastic wrap. Let steam for 10 to 15 minutes. Using your fingers or a paring knife, peel the peppers and chilies, then stem and seed them. Spread the peppers and chilies, skinned-side up, on paper towels and let drain for 10 minutes.
In a food processor, combine the walnuts, crackers, pomegranate concentrate, lemon juice, salt, cumin and sugar (if using) and process until smooth. Add the bell peppers and process until creamy, scraping down the sides as necessary. With the machine on, add the olive oil in a thin stream. Add the chilies, to taste, and process until smooth. If the spread is too thick, thin with 1 to 2 Tbsp. water.
Transfer the spread to a serving dish, sprinkle with the pistachios, red pepper flakes and a good pinch of cumin and drizzle with olive oil. Place the dish on a platter, arrange the pita chips alongside and serve right away. Alternatively, transfer the spread to an airtight container and refrigerate for at least overnight to allow the flavors to meld. Return to room temperature before serving.
From Unforgettable: The Bold Flavors of Paula Wolfert's Renegade Life, © 2017 by Emily Kaiser Thelin, M&P; originally published in Mediterranean Clay Pot Cooking by Paula Wolfert. Copyright (c) 2009 by Paula Wolfert. Used by permission of Houghton Mifflin Harcourt. All rights reserved.