Step 1Bring the rice, water and 1 teaspoon salt to a simmer in a large saucepan. Reduce the heat to low, cover and cook until rice is done, about 12 minutes. Immediately stir roughly with a spatula to crush many of the grains of rice. It will clump, but that's as it should be.
Step 2Heat the oil in a small skillet over medium heat. When hot, add the cashews and cook, stirring, until they begin to brown, about 2 minutes. Add the black mustard and cumin seeds and cook, continuing to stir, until the seeds sputter and pop, another 1 minute. Reserve half of the cashews and stir everything else into the rice.
Step 3Combine the yogurt and milk and stir it into the rice. The texture should be something like a good risotto: slightly clumping and just liquid enough to flow. Cover tightly and chill until ready to serve.
Step 4Dice half the radishes into one-fourth inch pieces and stir into the rice. Cut remaining radishes into one-eighth-inch slivers.
Step 5Season the rice to taste with salt and add a little more milk if necessary to correct the texture. Mound the chilled rice on a serving platter and scatter the reserved cashews and slivered radishes over top.