Combine cabbage, carrot, red onion, and 2 tablespoons kosher salt in a large bowl. Massage salt into vegetables. Set aside. Combine vinegar, sugar, allspice, pepper, and 1 cup water in a saucepan and bring to a boil over medium-high heat. Pour boiling liquid over cabbage mixture, then place a paper towel directly on top of surface, pressing down until it is saturated to keep the cabbage submerged. Let rest at room temperature until cool. Transfer to a sealed container and refrigerate until ready to use. Slaw will keep for about 2 weeks.
