Preheat the oven to 350 degrees F. Cut a lengthwise slit into each date and remove the pits. Stuff the cavity created in each date with an equal amount of pistachios. Gently squeeze the dates shut to seal the pistachios inside.
Insert a long, thin knife down the centre of the pork loin to create an end-to-end slit, 1 1/2 inches wide. Stuff the dates into the slit. Set the meat in a small roasting pan. Brush with the oil; season with salt, pepper and sage. Roast for 45 to 50 minutes, or until the centre registers 160 degrees F on a meat thermometer. Set the roast on a plate, tent with foil and allow it to rest 10 minutes.
Meanwhile, set the roasting pan on the stovetop over medium-high heat. Pour in the wine and simmer until reduced by half. Add chicken stock/flour mixture. Simmer until a lightly thickened sauce forms; season with salt and pepper. Slice the pork and serve the sauce alongside.
Note: Medjool dates are sold in the produce department; pistachios are in bulk foods.