Position racks in the middle and lower third of oven; preheat to 425 degrees F.

Place grapes on a rimmed baking sheet. Roast on the lower rack, shaking the pan occasionally to turn the grapes, until they are shriveled, 25 to 30 minutes.

Meanwhile, rub pork with salt and pepper. Heat oil in a large ovenproof skillet over medium-high heat. Add the pork and brown on one side, about 2 minutes. Turn the pork over and transfer the pan to the top oven rack. Roast the pork until just barely pink in the center and an instant-read thermometer registers 145 degrees F , 12 to 14 minutes. Transfer the pork to a cutting board to rest before slicing.

Place the pan over medium heat (use caution, the handle will be hot), add shallots and cook, stirring, until softened, 1 to 2 minutes. Add Madeira (or wine) and cook until reduced by half, 2 to 4 minutes. Stir in broth, thyme and mustard; bring to a simmer. Combine water and cornstarch in a small bowl and stir into the pan sauce. Cook until thickened, 30 seconds to 1 minute. Stir in the grapes. Serve the sliced pork with the grape sauce.

Shopping tip: Madeira, a fortified wine from the Portuguese island of Madeira, has a sweet, mellow flavor somewhat like sherry. Find it at liquor stores or in the wine section of the supermarket.