How to Make It
In a small saucepan, combine the 1 cup of water, the unseasoned rice vinegar, mirin and sugar with 1 1/2 tablespoons of salt and bring to a boil. Add the daikon and remove the saucepan from the heat. Let stand for 1 hour. Drain, reserving 2 tablespoons of the pickling liquid. Refrigerate the pickled daikon just until chilled.
Meanwhile, in a medium bowl, combine the tamarind with the 2 1/2 cups of boiling water and let soak for 10 minutes, or until it has broken up into a pulp. Mash the tamarind pulp with a potato masher and push it through a strainer set over a bowl.
In a medium saucepan, heat 2 tablespoons of the vegetable oil. Add the shallot, ginger and lemongrass and cook over moderate heat until softened, about 3 minutes. Add the tamarind pulp and ground cardamom and bring to a boil. Cook the sauce mixture over moderately high heat until reduced to 3/4 cup, about 10 minutes. Add the lime juice and honey and bring to a boil. Season the tamarind sauce with salt and pepper.
Heat a large skillet until very hot. Season the duck breasts with salt and pepper and add to the skillet, skin side down. Cook over moderate heat until the duck skin is deeply golden brown, about 15 minutes; pour off the duck fat as it accumulates in the skillet. Turn the duck breasts over and cook for 5 minutes longer for medium. Transfer the duck breasts to a cutting board and let rest for 5 minutes. Pour off the fat in the pan, add the tamarind sauce and bring to a boil.
In a large bowl, whisk 2 tablespoons of the reserved daikon pickling liquid into the remaining 1 tablespoon of vegetable oil and season the dressing lightly with salt and pepper. Add the daikon and the dates and toss to coat. Add the spinach or tatsoi and toss again. Mound the salad on plates. Thinly slice the duck breasts on the diagonal and set them next to the salad. Drizzle with some of the tamarind sauce and pass the rest at the table.