Preheat the oven to 375 degrees F. Combine plums, sugar, tapioca, salt, and nutmeg. Set aside while the oven preheats and you prepare crust.

Roll tart dough out on a floured section of parchment paper. Fold in half or quarters, unfold in a 10-inch buttered tart pan, gently press into the sides. Trim any excess dough from the edge. Starting in the center, push plums into the crust skin side down in a spiral. Finish with a squeeze of lemon and a liberal sprinkle of sugar.

Place on a cookie sheet and cook until the crust is golden and the plums are tender with juices bubbling, 40 - 45 minutes. Let tart cool for at least a half hour.

Slice and serve with a dollop of unsweetened whipped cream.