In a saucepan combine the apple cider vinegar, water, sugar, peppercorns, mustard powder, coriander and salt. Once sugar is dissolved, remove from heat and allow to cool to room temperature. Place the onions and mint in a bowl or jar, and pass the cooled pickling solution through a strainer, pouring it over the onions. Cover and chill for 12-24 hours. Can be stored in the refrigerator for up to a week.

In a saucepan, bring the salt and water to a boil. Pour the couscous into the boiling water, stirring to combine. Cover with a lid, remove from heat and let rest for 10 minutes. Fluff the couscous and chill for later use.

Assemble the salad by tossing the couscous with arugula, ¼ of the prepared pickled onions, and 1 tablespoon of the retained pickling liquid. Spread the couscous over the surface of a serving plate and layer with sliced oranges and grapefruit. Garnish with mint and pistachios, season with pepper, salt, and finish with a drizzle of olive oil. Serve chilled.