Combine the sugar, juice, stock, and Tamarind-Pasilla Paste in a heavy medium saucepan over medium heat. Cook and stir until the sugar dissolves. Then increase the heat so the mixture simmers. Cook, stirring often, until the liquid has thickened and reduces to a little less than 1 cup, about 20 minutes. Turn the heat to very low and keep the sauce warm while you cook the lamb.
While the sauce is cooking, heat a large ovenproof skillet over medium-high heat. Drizzle the lamb with the olive oil, sprinkle generously with salt and place in the skillet. Cook on each side for 5 minutes. Transfer to a cutting board and let rest for 3 minutes. Cut the lamb crosswise into 1/2-inch-thick slices.
Put the slices on the plates and spoon on the sauce. Sprinkle with the pomegranate seeds and serve right away.
Bring 1 cup water to a boil in a small saucepan. Line a plate with paper towels.
Heat 2 tablespoons of the olive oil in a medium skillet over medium-high heat until it begins to dance. Add the pasillas and fry on both sides until they're puffed up, about 15 seconds total. Transfer the chiles to the paper towels to drain. Put them in a small bowl, pour in the boiling water and let them soak until their soft, about 15 minutes. Drain the chiles and reserve them and the soaking liquid separately.
Discard the oil and wipe the skillet clean. Set it back over medium-high heat. In a large bowl, toss the garlic, tomatoes and onions with the remaining 1 tablespoon oil. Season with salt and pepper, toss gently and put them in the hot skillet. Cook until they're charred with visible black spots, about 7 minutes on each side. Transfer the vegetables to a clean bowl and let them cool to room temperature.
Put the tamarind pulp, chiles, 1/2 cup of the soaking liquid and the roasted vegetables in a blender or food processor and blend until smooth.
Store it in an airtight container in the refrigerator for up to a week or in the freezer up to a month.