Preheat the oven to 375°F. Line the cake pan with parchment paper (or grease & flour), and set aside.
Chop the rhubarb into 1 inch slices. Add into a small bowl, with 3 tablespoon sugar, lemon juice and lemon zest. (See notes)
Mix the cornstarch and water together, and set aside.
In a bowl, mix the flour, baking powder, salt and sugar.
In another bowl, mix the oil, non-dairy milk and cornstarch mixture, and mix well. Then add the rhubarb mixture.
Add the wet ingredients to the dry ingredients, and mix just until everything is incorporated. Transfer to the pan. If you want, you can add a decoration of rhubarb on top.
Bake for 30-40 minutes, or until a toothpick comes out clean (or with a few crumbs). Cool and enjoy!