- Wash and trim ends of rhubarb.
- Create rhubarb ribbons using a vegetable peeler or mandolin slicer to thinly slice rhubarb stalks lengthwise. You will need about 24 rhubarb ribbons for the lattice top. Save the remaining rhubarb for the gelée.
- Bring water and maple syrup to a rolling simmer in a saucepan. Roll rhubarb ribbons up and working in batches, blanch them in the maple syrup-water reduction for about 30 seconds to 1 minute. Transfer rhubarb ribbons to a towel lined baking sheet using kitchen thongs and allow them to drain while you blanch the next batch.
- Carefully unroll the ribbons. Create a lattice or basket-weave pattern by weaving the rhubarb ribbons over and under each other on top of a sheet of wax paper. Use the tart pan as template to cut away the edges of the lattice with a sharp knife, creating a 9" round circle which will lay on top of the finished tart. Save the rhubarb trimmings for the gelée.
- Chop the uncooked rhubarb pieces. Reduce the rhubarb cooking liquid at a low boil and add the raw rhubarb and lattice trimmings to the liquid, simmering until tender, about 1 minute.
- Transfer rhubarb and cooking liquid to a blender. Add strawberries, lime zest, agar agar, and vanilla extract and purée until smooth.
- Return purée to the saucepan and cook on medium heat, stirring constantly for 5 minutes to activate the agar agar. Strain warm gelée through a fine sieve to remove any lumps that may have formed from the agar agar. The gelée will continue to thicken and set as it cools.
- Pour gelée into cooled tart crust and spread into a even layer with an offset spatula.
- Transfer the tart to the freezer to allow the gelée layer set up while cashew cream is prepared.