Heat up fish stock to a simmer, allow to cool for 3 minutes, dissolve in gelatin sheet. Cut escolar into 2 broad thin sheets, measuring approximately ½” thick X 2” wide X 6” long. Cut tuna into 1 broad thin sheet, measuring approximately ½” thick X 2” wide X 6” long. ‘Paint’ one side of each piece of escolar with the jellied fish stock. ‘Paint’ both sides of the tuna piece. Assemble ‘sandwich’ by ‘sandwiching’ the tuna piece between the two escolar pieces. Making sure that the ‘painted’ sides of the escolar are touching the tuna and the ‘unpainted’ side of the escolar is the outside of the ‘sandwich’. Wrap the sandwich tightly in plastic wrap and refrigerate for a minimum of 4 hours.