Peel the 16 oz (455g) cucumbers and halve them lengthwise. Scoop out the seeds with a melon baller and slice them crosswise about 0. 1 inch (3 mm) thick.
Bring a pan of water to a boil, add the cucumbers, and blanch them for exactly 2 minutes.
Pour through a strainer, shake off the excess water, and immediately plunge them into a bowl of ice-cold water to stop the cooking.
Strain once more and gently pat them completely dry with paper towels.
Combine the 1. 0 cup (8 oz / 225g) plain yogurt, 1. 0 cup (8 oz / 225g) sour cream, 0. 5 to 1. 0 tsp (0. 1 oz / 3g) salt, 1. 0 tbsp (0. 1 oz / 3g) parsley, 2. 0 tbsp (0. 1 oz / 3g) dill, 2. 0 tbsp (0. 5 oz / 14g) walnuts, 0. 33 cup (1. 2 oz / 35g) shredded coconut, and the slivered chili in a large mixing bowl.
Whisk everything together with a fork until the mixture is beautifully smooth and creamy, then gently stir in the dried, blanched cucumbers.
Heat the 2. 0 tbsp (1. 0 oz / 28g) coconut oil in a small pan over moderate heat.
Once the oil is hot (but not smoking), carefully add the 1. 0 tsp (0. 1 oz / 3g) black mustard seeds.
Fry them briefly until the seeds begin to sputter and turn a grayish color (use a spatter screen if the oil is jumping). Immediately pour this hot oil and fried seed mixture directly into the yogurt-cucumber bowl and stir well to distribute the flavors.
Refrigerate the finished dish for 1 to 2 hours; this chilling time is crucial as it allows the seasonings to release and meld their flavors.
When ready to serve, transfer the cold raita to a flat serving dish or individual bowls, and garnish beautifully with the 0. 5 oz (14g) of fresh watercress sprigs.
Dry the Cucumbers Thoroughly: The biggest mistake people make with raita is letting it get watery.
After blanching and shocking the cucumbers in ice water, use a paper towel to pat them completely dry. This ensures your yogurt-cream base stays luxuriously thick.
Chill Your Serving Bowls: Since this dish is best served very cold, place your serving bowls in the refrigerator for 20 minutes before plating. This keeps the raita refreshing, especially if you are serving it alongside piping hot parathas or curries.