Make the dressing: Combine all dressing ingredients (basil, olive oil, lime juice, garlic, salt, red pepper flakes, and water) in a food processor. Pulse until smooth-ish. If it’s too thick, add water one tablespoon at a time.
Cook the couscous: Bring 1 cup of water to a boil, stir in couscous, cover, and remove from heat. Let it sit for 5 minutes, then fluff with a fork and let it cool.
Cook the sweet potatoes: Place diced sweet potatoes in a skillet with 1/2 cup water and a pinch of salt. Cover and simmer for about 10 minutes, or until fork-tender. Drain and set aside to cool.
Assemble the salad: In a large bowl, toss the cooled couscous, sweet potatoes, black beans, spinach, feta cheese, red onion, and pistachios (if using) with the prepared lime-basil dressing. Taste and season with black pepper.
Serve or store: Serve immediately as a main course or side dish. Alternatively, store the components separately for make-ahead lunches throughout the week.