For the Uni Foam:
Cook the rice in nage at low heat, simmering covered for 30 minutes until it retains a pudding-like consistency. Pour the pudding into a blender and process at high speed, incorporating the eggs, egg yolk, butter, and urchin at the end. Blend until very smooth, then season.
In a bowl over a simmering, whisk the mixture until warm enough so that the eggs are cooked. Do not overcook, or the eggs will scramble. Place into an iSi whipper and reserve at 160°F.
For the Kanten Sheets:
Bring the nage to a boil, then whisk in the gelatin for 1 minute over medium heat. Pour into ½-sheet pan and refrigerate until firm. Cut into 1½-inch-by-2½-inch sheets.
To Assemble and Serve:
Place cold nage in a medium-size pot and add the shrimp. Heat over low flame until simmering, then remove shrimp. Whip sour cream with shallots, chives, lemon juice, salt, and pepper.
Shake the iSi whipper before serving, then foam the oursinade into a soup plate. Place a tablespoon of sour cream in center, 2 shrimp on top, and encase a tongue of sea urchin between two Kanten Sheets. Garnish with chervil.