In a small bowl, whisk together sage, oil, salt and pepper. Spread evenly on pork loin. Insert a steel pipe horizontally into the pork loin, creating a hole in the centre. Set pork loin aside.

Preheat oven to 450°F. Heat canned peaches in a small saucepan until simmering. Add cornstarch and agar-agar and continue to cook until the mixture thickens. Remove from heat and cool slightly before pouring the mixture in a piping bag and piping it into the hole in the centre of the pork loin. Place the pork loin in a prepared roasting pan and roast, uncovered, for 20-25 minutes. Reduce heat to 325°F and continue cooking until a thermometer inserted into the thickest part of the pork loin reaches 155°F (68°C), about 10-20 minutes. Remove from oven and cover loosely with tented aluminium foil. Let rest for 10-15 minutes before slicing.