Start by soaking the cashews. While they are sitting for 60 minutes, make the crust.
In a food processor or blender pulse the graham crackers and vegan butter. The dough should be wet and crumbly. Place 2 Tablespoons of the crust in the bottom of each mason jar and press to coat evenly. I had good luck with a vitamin bottle for pressing down the crust. Set the jars in the freezer to firm for about 30 minutes.
Add the cashews, lemon juice, coconut oil, coconut milk, and half the maple syrup to a blender and puree until very smooth. Pour the mixture into each mason jar over the crust with ~1/2 inch left on the top (about 1/4 cup filling in each jar). Place back in the freezer to set for 60 minutes.
Mix the strawberries, rhubarb and the other half of the maple syrup in a medium mixing bowl. Let the fruit sit for 60 minutes so flavors can mingle.
Remove the jars from the freezer and top with 2-3 Tablespoons of fruit on top. Garnish with a sprinkle of grated lemon if desired.