For the poached rhubarb: Cut the rhubarb into rectangles measuring 3mm/.12in wide by 17.5cm/7in long by 3mm/.12in high. Reserve in a quarter hotel pan.
Bring the grenadine to a boil and pour it over the rhubarb.
Quickly wrap the pan and let the rhubarb poach in the liquid. Leave it in the refrigerator overnight.
The following day, make the gelée, or reserve the rhubarb in the liquid for up to 2 weeks in the refrigerator.
Lightly spray a very flat sheet pan with nonstick oil spray; spread it out evenly with a paper towel. Place a sheet of acetate over the sprayed surface and smooth it out with a clean paper towel to eliminate any air pockets.
Place a stainless-steel rectangular frame measuring 8.75cm/3.5in wide by 17.5cm/7in long by 1.25cm/.5in deep on top of the acetate-lined sheet pan. Seal the exterior border base with rolling fondant (it works like caulk to prevent the liquid from seeping out of the frame).
Take the rhubarb out of the poaching liquid and pat it dry.
Line up the rhubarb rectangles horizontally inside the prepared frame, spacing them out evenly and ensuring that they are straight.
Make the gelée by combining the elderflower syrup, water, and gelatin sheets in a bowl and placing them over a hot water bath, stirring gently to melt them together.
Let the gelée cool to about 27°C/80°F, and then gently and slowly pour it into the frame. Fill it up as much as possible.
Transfer the gelée setup to the refrigerator to set. Once it has set, pass the tip of a paring knife around the inside of the frame and take it off.
Dip a long, thin slicing knife into a bain-marie filled with hot water and dry it with a paper towel. Cut out 1-cm/.4-in-wide rectangles across the rhubarb. Cut across the 8.75-cm/3.5-in width of the gelée to obtain gelée-and-rhubarb rectangles measuring 1cm/.4in wide by 8.75cm/3.5in long by 1.25cm/.5in deep.
Reserve the rectangles in an airtight container in the refrigerator. Discard after 2 days.