Wash the kale thoroughly and break off the stems where they meet the leaf. Chop the stems into dice that are the same size as the onions.
Set a cast-iron frying pan on a burner and turn to medium. Pour about 1 1/2 cups of water into the pan and bring to the boil. Have a bowl of ice water ready beside the stove.
Cook the kale leaves in two or three batches until wilted but still bright green. With a pair of kitchen tongs, remove the kale and plunge it into the ice water. Repeat until all the kale is cooked. Pour the cooking water into a heat-proof bowl or cup, and reserve.
Once the remaining skiff of water is evaporated from the pan, pour in two tablespoons of oil. Add onion and kale stems and a couple of tablespoons of reserved cooking water. Sauté onion and stems, adding cooking water as necessary, until onions are translucent.
Add spices and garlic to the frying pan and sauté for another 2 or 3 minutes. Remove from heat and allow to cool to room temperature.
While the onion is cooking, squeeze the kale leaves between your hands to remove as much moisture as you can and place on a cutting board. Chop coarsely and transfer to a medium-sized bowl.
Once the onion-kale mixture has cooled, add to the bowl. Stir in the yogurt and the kosher salt. Taste and add more salt as necessary. Refrigerate for a couple of hours to allow the flavours to blend.
Serve with triangles of pita or naan bread and chopped vegetables, with barbecued chicken or vegetables, or on a platter with a bunch of other dips and some olives. Will keep in the fridge for up to one week.