A combination of cornstarch and coconut milk could also be used to thicken a lemon bar, but I haven't tested this alternative yet. Agar powder is another popular thickener, which is used in my Vegan French Silk Pie. You'd have to heat either of these options in a saucepan to thicken, or bake the filling in the oven until set. (I'll report back when I have a cashew-free alternative.)