Prepare the duck breasts: rinse and pat them dry. Using a sharp knife, score the duck skin in a crisscross pattern, taking care to cut only through the skin and fat, avoiding the duck flesh. Grind the coriander and peppercorns in a spice grinder or coffee mill, or crush well using a mortar and pestle. Add the garlic and salt, spin or mash to a coarse puree, and rub into both sides of the duck, working some of the paste into the flesh between the score marks. Set aside while you start the sauce. Preheat the oven to 300°F. Prepare the sauce: rinse, quarter, and peel the quinces. Using a very sharp knife, cut out the core, seeds, and any other hard bits. Cut the quinces into large chunks. Pulse the walnuts in a food processor to a mixture of fine and coarse crumbs. In a large heavy Dutch oven or casserole, heat the oil over medium heat until it shimmers. Add the onions and sauté, turning occasionally as they soften and then turn golden. Adjust the heat to medium-high and add the quinces. Cook until the fruit begins to speckle a deep caramel in places, lifting and tossing to avoid burning. Add the garlic, sprinkle with salt and pepper, and cook for 3 minutes to blend the flavors. Stir in the ground walnuts and briefly sauté, stirring, until their aroma arises. Stir together the pomegranate juice and pomegranate molasses, and add along with the broth and dates. Bring the mixture to a boil, then cover and transfer to the oven, occasionally giving it a stir as it cooks. (You can simmer the sauce on top of the stove, if you prefer. I find the oven heat gentler and more even, causing less evaporation, and besides, I usually need the extra burner space on the stove.) When the sauce has been in the oven for about 45 minutes, choose a very large, heavy, ovenproof skillet (cast-iron is ideal here; if necessary, use 2 skillets or work in batches) and film it with oil. Place the skillet over medium heat until hot. Add the duck breasts, skin side down. Cook, shaking the pan occasionally or sliding a spatula under the breasts if they threaten to stick. Press down on the breasts with a spatula now and then to release more fat, and pour out the rendered fat as it accumulates; if the fat rises up to the duck flesh, its high temperature may toughen the meat. Continue cooking until the skin is well bronzed, about 10 minutes. Pour off any remaining fat from the pan, turn the duck skin side up, and put the skillet in the oven alongside the sauce. (Or, if you sautéed the duck breasts in batches or in 2 skillets, you can place all of them in one large baking pan.) Slow-roast until the duck breasts are cooked to the desired doneness, 10 to 15 minutes, or until they feel springy to the touch and a thermometer inserted horizontally into the center of a breast registers 135°F for medium-rare or 140°F for medium. Transfer the duck to a cutting board and let rest for 5 minutes. Remove the sauce from the oven and place the pot over medium heat. Add the lemon juice and cook for 2 minutes to blend the flavors. Taste and adjust the lemon, salt, and pepper. Cut the duck on the diagonal into 1/2-inch-thick slices. It is easiest to do this if the thick skin side is on the bottom and you cut directly into the duck flesh. Spoon fluffy cooked rice onto a serving platter and nap with plenty of sauce and chunks of quince. Fan the duck slices decoratively on top, ladle some additional sauce over, and scatter the pomegranate seeds, if using.