Preheat oven to 350F/180C. Line an 8x8 baking pan with parchment paper to fit snug along the bottom with flaps up the side.
Make the crispy crust: Pulse the oats in a high-speed blender until flour-like consistency. Add to a small mixing bowl, with the almond flour, sea salt, agave and melted coconut oil. Mix to combine.
Transfer the cookie crust to the baking tray and flatten with a spatula to make an even, solid base. Bake in the oven until golden, 8 to 10 minutes. Remove and set aside. Keep the oven on.
Make the lemon filling: In a medium saucepan, whisk together the lemon juice, sugar, and arrowroot flour. Scoop 1 1/4 cup of the canned coconut milk into your saucepan using the thick white portion only (avoiding using the coconut water). Add a pinch of turmeric and sea salt and bring to medium heat. Cook the mixture, stirring often, until smooth and thickened into a custard consistency and gelatinous texture, 10 minutes.
Transfer the lemon filling to the baking pan and smoothen evenly with a spatula. Bake until the edges are slightly browned, 15 minutes. Let cool for 30 minutes. Then transfer the bake to the fridge to chill entirely, at least 2 hours, but preferably overnight.
Lift the dessert out of the dish using the parchment flaps, and dust with powdered sugar. Cut into 16 squares.
Filling takes about 2 hours to chill completely, but it's preferable to leave overnight to ensure they're entirely cool before cutting.Lemon bars will keep in the fridge for up to 1 week. Store in an air-tight container. They can be frozen for up to 2 months. Reserve the coconut water from the canned coconut milk for smoothies. Nutrition information is a rough estimate.