Pre-heat the oven to 350 F degrees. Using the coconut oil, thoroughly oil a 9-inch tart pan with a removable bottom and set it aside.
Place walnut halves, oats, brown rice flour, and salt into the bowl of a food processor. Grind for 20 seconds or until coarsely ground.
Transfer to a bowl and stir in the spelt flour. Pour in the coconut oil, maple syrup, and vanilla extract. Using a fork, mix until thoroughly incorporated. It should moist, but no sticky.
Transfer the dough onto the prepared tart pan. Either using your fingers or the back of a spoon, evenly press the dough into the tart pan until it is about ¼-inch in thickness. Here, you want to make sure that the tart dough reaches all the way to the edges of the tart pan. Trim any excess dough. Prick the bottom of the crust several times with a fork and bake for 18 minutes or until it turns golden-brown. Remove from the oven and let it cool on the kitchen counter.
To make the filling: Place 1-cup apple juice and agar flakes in a small heavy-bottom pot. Whisking frequently, bring it to a boil over high heat. Cover the pot, turn the heat down to low, and let it simmer for 5 minutes. At the end of 5 minutes, agar flakes should be completely dissolved.
Meanwhile, in a small cup whisk together the arrowroot powder and the remaining 2 teaspoons of apple juice until it is completely dissolved. Slowly whisk it into the simmering apple juice and agar mixture. Turn the heat up to medium-high and whisk constantly until it thickens. This should take no more than 2-3 minutes. Set it aside to cool for 5 minutes. Do not let it set, if it does, return it to medium heat and stir it until it is in liquid form again.
Place sliced peaches and blueberries in a large bowl and drizzle it with the maple syrup and vanilla extract. Give it a gentle stir. Pour the warm agar mixture over the fruit; working quickly, use a rubber spatula to coat the fruit completely. Transfer the fruit mixture into the tart and distribute it evenly. Pour any of the remaining juices over the fruit. Place it in the fridge for at least 30 minutes.
When ready to serve, sprinkle the tart with chopped pistachio. Serve each slice with crème fraiche or a scoop of your favorite ice cream.