1 1/2 pounds sweet potatoes, regular waxy potatoes, or Yukon gold potatoes
1 medium red onion
2 tsp coarse salt
2 tbsp vegetable oil
2 tsp ground cumin
1 tsp ground coriander
1/2 tsp ground red pepper
1/2 tsp freshly ground black pepper
1/2 cup plain yogurt
2 tbsp fresh lemon juice
Leaves from 1 bunch of cilantro
Leaves from 1 bunch of mint
1. Cover the potatoes with cold water in a pot and bring to a boil over high heat. Immediately reduce the heat to low and simmer gently until the potatoes feel soft all the way through when poked with a small knife, about 30 minutes. Don't over cook them or they will fall apart when you put the salad together. Drain the potatoes, let them cool at room temperature, and chill them for several hours or overnight in the refrigerator.
2. Slice the onion as thin as possible and rub the slices with the coarse salt until you can't feel the salt on your fingers. The salt softens the onion and helps its texture meld with that of the sweet potatoes. Drain the onions in a colander for 20 minutes, quickly rinse the onion under cold water, and squeeze the softened onion slices in your fingers to extract as much of their juice as you can.
3. Combine the vegetable oil with the cumin and coriander in a small sauce pan. Cook the spices over medium heat while stirring with a wooden spoon until you smell their fragrance, about 2 minutes. Stir in the red and black pepper, yogurt and lemon juice.
4. Peel the sweet potatoes by pulling away the skin with your fingers or with a paring knife. Slice the sweet potatoes into rounds about 1/3 inch thick.
5. Chop the cilantro and mint leaves and stir them into the yogurt spice sauce. Arrange the sweet potato slices in alternating layers with the onion and the yogurt/spice sauce in a salad bowl or baking dish, finishing with a layer of sauce. Serve at room temperature or slightly cool.