For the Tomato Water: Blend the ripe beefsteak tomatoes with the salt until puréed, about 2 minutes. Place the puréed tomatoes in a large fine-mesh strainer set over a stainless steel bowl or bain marie and let gravity extract the water from the tomatoes overnight.
Place a 12.5cm / 5in square by 1.75cm / .5 in mold on an acetate sheet on a flat surface such as a sheet pan or a sheet of Plexiglas.
For the Tomato Jelly: Over medium heat, boil 99g / 3.49oz of the tomato water with the agar-agar and salt. Combine with rest of the liquid off the heat. Pour into the prepared mold. It may seep, so it is a good idea to seal the mold by piping melted chocolate around it and letting it set before pouring in the jelly. Let the jelly set in the refrigerator.
Take the chocolate off the tomato jelly mold with a paring knife. Wipe the surrounding surface clean of any chocolate so none of it combines with the jelly.
Reserve chilled and covered in plastic wrap. Discard after 2 days.