Preparation

• Position a rack in the center of the oven, and heat the oven to 400°F. Put a rack over a rimmed baking sheet and set aside. Pat the pork dry with paper towels, and season with 1/2 tsp. salt and 1/4 tsp. pepper. In a large nonstick ovenproof skillet, preferably cast iron, heat the oil over medium-high heat. When the oil shimmers, add the pork and let cook on one side until a golden crust forms, about 3 minutes.
• Repeat, rolling the pork as needed, until seared on all sides. Add the grapes, transfer the skillet to the oven, and roast until an instant-read thermometer inserted into the pork reads 140°F, 5 to 8 minutes. Remove the tenderloins from the oven, and transfer to the rack on the baking sheet. Tent lightly with foil, and set aside. continue roasting the grapes until dark and slightly shriveled, 7 to 8 minutes more. Using a slotted spoon, remove the grapes.
• Transfer the skillet to the range. Heat over medium heat. Add the shallot and thyme sprig, and cook, stirring frequently, until the shallot begins to turn translucent, about 1 minute. Add the wine, increase the heat to medium high, and cook until reduced by about half, stirring occasionally and scraping up the browned bits on the bottom of the pan, 1 to 2 minutes.
• Add the broth and cook, stirring occasionally, until reduced by about half, 1 to 2 minutes. Discard the thyme sprig. Whisk in the butter, one piece at a time, until each piece is incorporated. Season to taste with salt and pepper. Slice the pork crosswise into 1/2-inch-thick slices, and serve immediately with the roasted grapes and the sauce. Garnish with additional fresh grapes and the basil.