2 medium Italian frying peppers, cut into small dice
1 teaspoon ground cumin
1 teaspoon ground coriander
1/4 teaspoon cayenne pepper
Preheat the oven to 450°. Heat a large ovenproof skillet until very hot. On a plate, rub the roast all over with the soy sauce and then with 1 tablespoon of the olive oil; season with salt and pepper. Add the roast to the skillet, fat side down, along with any extra oil from the plate. Cook over moderately high heat, turning occasionally, until browned all over, 4 minutes. Turn the roast fat side up and roast in the oven for 25 minutes, or until an instant-read thermometer inserted in the thickest part of the meat registers 115° for rare. Transfer the roast to a carving board to rest.
Meanwhile, spread the walnuts on a baking sheet and toast for about 5 minutes, or until lightly browned; let cool. Coarsely chop the nuts.
In a medium skillet, heat the remaining 1/2 tablespoon of olive oil. Add the frying peppers, cumin, coriander and cayenne, cover and cook over moderately low heat until the peppers are barely softened, about 5 minutes. Let cool.
In a medium bowl, combine the yogurt, garlic, walnuts, frying peppers and tomatoes and season with salt and pepper. Thinly slice the roast and serve with the walnut sauce.
Variations Use the spicy walnut sauce on chicken or lamb, or serve as a dip for pita bread.