Brisket With Pomegranate-Walnut Sauce and Pistachio Gremolata

Season brisket all over with salt and pepper. Transfer to a large 2-gallon resealable plastic bag or bowl (use roasting pan only if necessary). Purée garlic, walnuts, honey, and 1 cup pomegranate juice in a blender until very smooth. Add remaining 2 cups pomegranate juice and blend until smooth. Pour marinade over brisket. Seal bag or cover bowl tightly with foil. Chill, turning occasionally, at least 24 hours or up to 48 hours. Transfer brisket and marinade to roasting pan, cover tightly with foil, and let sit at room temperature 1 hour. Preheat oven to 275 °F. Bake brisket, covered, until meat shreds easily with 2 forks, about 5 hours; if meat is still tough, continue cooking, covered, 1 hour. Transfer brisket to a cutting board and cover loosely with foil. Transfer cooking liquid to a saucepan and spoon off fat from surface. Cook over medium-high heat, skimming off fat and foam as it surfaces, until reduced by two-thirds (you should have about 2 cups sauce). Season with salt and pepper if necessary.