Place the cashews in a small bowl and cover with boiling water. Soak for 1 hour, then drain. (Alternatively, soak the cashews in room-temperature water to cover for at least 6 hours or overnight, then drain.)
Preheat the oven to 350°F (180°C).
In a food processor, pulse the oats until they reach a flour-like consistency. Transfer the oat flour to a small bowl.
Add the walnuts to the food processor and pulse into a fine meal. Add the oat flour, almond flour, coconut oil, maple syrup and salt. Pulse until well combined, 15 to 30 seconds.
Transfer the dough to a 9-inch round tart pan with removable bottom and press it evenly into the bottom and up the sides. Using a fork, poke 10 to 12 holes in the base of the dough. (This will stop the base from lifting as it cooks. There’s no need to use pie weights with this crust.) Place on a baking sheet and blind-bake until golden brown, about 10 minutes. Let the crust cool on a wire rack for at least 20 minutes.
In a high-speed blender, combine the drained cashews, coconut milk, lemon zest, lemon juice, maple syrup, ⅛ teaspoon of the turmeric, and salt. Blend on medium-high speed until smooth and combined. Add up to ⅛ teaspoon more turmeric for colouring, if desired. (Note that the colour will deepen when the filling is heated.) Add the agar-agar powder and blend on low speed until combined, 15 to 20 seconds.
Pour the filling into a medium saucepan. Over medium heat, stir often with a spatula until the mixture has thickened to a pourable custard consistency (just before it reaches boiling) and there is a bit of resistance, 7 to 10 minutes. Working quickly, pour the filling into the blind-baked tart shell. It will begin to thicken immediately. If the filling is uneven, gently jostle the tart back and fork on the counter to smooth the top. Pop any air bubbles with a toothpick.
Let the tart cool completely in the pan on a wire rack at room temperature. Then cover with plastic wrap (it’s okay if it touches the filling) and chill in the refrigerator for at least 3 hours, preferably overnight. To serve, gently remove the sides of the pan. If desired, garnish the tart with thinly sliced lemon rounds, fresh raspberries, and a sprinkle of coconut flakes.