Blanch English peas: Bring a small saucepan of salted water to a boil, add peas, and cook 2 minutes. Drain and immediately plunge into ice water to cool.
Make preserved onions: In a bowl, combine red onion slices, 2 tablespoons lemon juice, 1/2 teaspoon salt, and 1/2 cup water. Soak 20 minutes, then drain and blot with paper towels.
Cook couscous: Bring 1 1/2 cups water to a boil. Add couscous, salt, and 1 tablespoon oil. Reduce heat, cover, and simmer 10 minutes until al dente. Fluff with a fork.
Make marinade: Whisk 2 tablespoons lemon juice, lemon zest, remaining 1/2 teaspoon salt, za’atar, garlic, and olive oil until smooth.
Combine salad: In a large bowl, mix couscous, preserved onions, blanched peas, arugula, and remaining 1 1/2 tablespoons lemon juice. Toss with half the marinade, then stir in pistachios and mint. Adjust seasoning if needed.