Place a 25cm / 10in by 38cm / 15in by 2 mm / .08 in Plexiglas frame on a half sheet pan.
Heat the water to 80°C / 176°F. Take the water off the heat and add 238g / 8.4oz of the shiso leaves. Cover and steep for 30 minutes, then strain out the leaves. Reserve the water and season with salt to taste.
Cut the remaining shiso leaves into a fine chiffonade.
Mix the shiso water with the agar-agar. Bring to a boil while stirring constantly with a whisk. Let boil for 10 seconds. Take the pot off the heat and add the gelatin. Stir until the gelatin is melted.
Pour the liquid into the prepared frame.
Sprinkle the shiso leaf chiffonade evenly throughout the entire surface of the liquid, working quickly so that the water doesn’t set before the leaves are added. Let it set in the refrigerator.
Cut into 5-cm / 2-in squares using a metallic cutter or knife. Discard leftover jelly after 2 days.