Steam the couscous: Add the dry couscous and salt to a large bowl. Pour in boiling water (check the package for the exact ratio, but a 1:1 couscous to water is standard). Stir once, then cover the bowl with a plate or lid. Let it steam for 10 minutes, until the liquid is fully absorbed.
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Make the chimichurri: Remove the thick stems from the parsley, then finely chop the leaves with a knife. Add them to a jar along with the olive oil, red wine vinegar, minced garlic, dried oregano, salt, and chili flakes. Stir to combine and let it sit for at least 10 minutes so the flavors can meld.
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Add-ins: Finely dice the cucumber and thinly slice the green parts of the scallions. If using unshelled pistachios, shell them by hand, then give them a rough chop.
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Corn: Drain the corn, then sauté it in a pan with a little olive oil over medium-high heat until golden.
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Fluff the couscous with two forks to separate the grains. Add the lime zest, diced cucumber, sliced scallions, chopped pistachios, sautéed corn, and cooked chickpeas (if using). Toss gently to combine.
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Assemble the salad: Fluff the couscous with two forks to separate the grains. Add the lime zest, diced cucumber, sliced scallions, chopped pistachios, sautéed corn, and cooked chickpeas (if using). Toss gently to combine.
To serve immediately: Add the chimichurri to the salad and toss until evenly coated. For meal prep: Divide the salad into 4 individual containers and the chimichurri into 4 salad dressing containers. Refrigerate for up to 4 days. Add the chimichurri just before serving and shake to combine.