Preheat oven to 400°F.
Wrap the beets in aluminum foil and bake for about 75 minutes, or until tender. Refrigerate the beets for 4 to 6 hours. Peel and chop the roasted beets into ½-inch cubes and set aside.
Whisk the cumin, black pepper and salt into the yogurt in a large bowl. Gently fold the beets into the yogurt along with the cashews, ginger, cilantro and lemon juice.
Heat the olive oil in a small pan over medium-high heat and fry the mustard seeds until they pop. Immediately pour the oil and seeds over the yogurt mixture and fold together. Refrigerate until ready to serve.