To make the crust, mix butter, brown sugar, cinnamon, salt, and pecans in a medium bowl. Add flour and stir until just blended. Press the dough evenly over the bottom and up the sides of a 9 1/2-inch tart pan with a removable bottom. Generously prick the bottom and sides with the tines of a fork. Bake in the lower third of an oven preheated to 350° F until the crust is pale golden, about 20 to 25 minutes. Cool completely.

To roast grapes, toss grapes, olive oil, sugar, salt, and rosemary on a rimmed baking sheet lined with foil. Roast in oven preheated to 375° F until just starting to collapse and split, about 15 minutes. Remove from oven. Discard rosemary sprigs (it's okay if a few leaves remain with the grapes) and drizzle with vinegar. Toss gently and set aside to cool to room temperature.

For the cream filling, whisk together mascarpone, cream, and sugar with an electric mixer until it just holds stiff peaks. Scrape mixture into the tart crust, spreading it evenly to the edges. Spoon roasted grapes over the cream filling.

If desired, drizzle slices with remaining juices and garnish with sprigs of rosemary.