Bring a large pot of salted water to a boil. Add couscous and cook until just tender, 4 to 6 minutes. Drain. Meanwhile, in a small dry skillet, toast cumin seeds until fragrant, 1 to 2 minutes. Lightly crush them using a mortar and a pestle (or use the flat side of a heavy knife and a cutting board). Add to a bowl with the warm couscous, vinegar, salt and pepper and toss well. When the couscous is cool, add remaining ingredients and mix well. Taste and add more salt, lemon juice or oil if needed. Garnish with pistachio nuts if desired.