1. Halve cucumber lengthwise; use spoon to scrape out seeds. Slice very thin.
2. Make the raita: Stir together cucumber and 1/4 teaspoon of the salt in a colander and let drain for 15 minutes; pat dry with paper towels. Meanwhile, stir together cucumber, yogurt, radishes, minced mint, garlic, lemon zest, 1/4 teaspoon of the salt, and 1/4 teaspoon of the pepper. Store covered in the refrigerator until ready to serve.
3. Reserve 6 spinach leaves; store covered in refrigerator for up to 2 days. Heat oil in a wok or large deep skillet over medium-high heat until hot but not smoking. Add remaining spinach, 1/4 teaspoon of the salt, and 1/8 teaspoon of the pepper, and stir-fry in batches, turning, just until wilted. Remove from wok and cover with foil.
4. Heat broiler. Season salmon with remaining salt and pepper; broil on a broiler pan 5 to 6 inches from heat source for 5 to 7 minutes, or until opaque and just cooked through.
5. Set aside 2 of the fillets; let sit until at room temperature cover and store in fridge for up to 2 days.
6. Arrange cooked spinach on 4 plates. Top each serving with a fillet, then with 1/4 of raita, and garnish with mint sprigs. Serve hot.
How kids can help: Measure and mix ingredients for raita; top fillets with raita; sprinkle with mint.