Step 1Using paper towels, pat any moisture from the surface of the pork medallions. Season the pork with one-half teaspoon salt and one-fourth teaspoon pepper.
Step 2In a heavy 10-inch skillet, heat the butter and oil over high heat. Add the pork medallions to the skillet and sear until nicely browned, 2 minutes. The meat should release easily from the pan when turning. Brown the other side for 30 seconds to 1 minute so the meat is still slightly underdone. Remove the pork medallions from the skillet to a plate, cover and keep warm.
Step 3Reduce the heat to low. Add the shallots to the skillet and quickly saut? for 1 minute, then add the garlic and saut? 30 seconds. Add the grapes and saut?, stirring up any browned bits from the bottom of the pan. Saut? the grapes for 2 minutes until they begin to collapse. Add the red wine. Bring to a simmer over medium heat and cook until the wine is reduced by half. Season with one-fourth teaspoon salt and a pinch of pepper, or to taste.
Step 4Place the medallions back into the skillet along with any drippings and simmer, spooning the sauce over the pork medallions as they warm, 1 to 2 minutes or until just pink in the center. Remove the meat from the skillet (leave the sauce in the pan) and arrange on a platter or on serving plates.
Step 5Add the butter to the pan, tilting the pan and swirling the butter until the sauce is slightly thickened. Spoon some of the sauce with the grapes over each serving.