Sprinkle 1 teaspoon of kosher salt in the bottom of a 500 ml (2 cup) mason jar. Pack in sage leaves until half full. Sprinkle another 1/2 teaspoon salt over leaves. Continue to layer in leaves until the jar is full. Pour in olive oil until the leaves are completely covered. Make sure no leaves poke through the oil. Cover tightly and leave in a cool place indefinitely.