Preheat your oven to 400℉ and prep a 9-inch round tart/pie pan with non-stick spray.
In a food processor, combine the graham crackers, salt, and coconut sugar. Pulse a few times to mix together. Add in the melted vegan butter and pistachio oil (or more vegan butter if you don't have any) and pulse again until it resembles wet sand.
In a small bowl, combine the coconut flakes with the graham cracker mixture and press it firmly into the bottom of your tart pan. Bake until just set and golden brown, or about 5-8 minutes.
Meanwhile, juice your grapefruits. Combine the coconut sugar and juice in a small saucepan and heat over medium heat. Cook for several minutes until thickened and bubbly. Add in your flax eggs and cornstarch and mix well. Cook for 2-3 minutes, stirring frequently, until the mixture is thickened and sticks to the back of your spoon.
Remove the mixture from heat and stir in the pats of vegan butter until melted. Pour into the cooled tart crust and smooth the top with a silicone spatula. Cover with plastic wrap and place in the fridge for at least four hours or overnight.
Once you are ready to serve, top with chopped pistachios and coconut flakes. Store any leftovers covered in the fridge for up to three days.
