Preheat the oven to 170C and grease a 20cm loose-bottomed tart tin.
Place the oats and ground almonds in a food processor along with the melted coconut oil and Natvia and pulse until a flapjack-like mixture forms. Tip into the tin and press across the base and up the sides evenly. Set aside.
Place the rhubarb chunks in a pan with the orange juice and Natvia. Cook on a high heat for a few minutes then turn down and simmer for 10 minutes until softened and excess liquid has evaporated. Set aside to cool a little then spoon over the tart case, scattering the raspberries over.
Mix together the flaxseed and water and set aside for 10 minutes to form a gel. Then add into a bowl with the ground almonds, Natvia, baking powder, melted coconut oil, almond milk and extract and whisk well.
Spoon this over the fruit mixture to cover and smooth out evenly. Scatter over the flaked almonds and bake for 35-40 minutes (cover with foil halfway) until golden and firmed up. Leave to cool in the tin then pop out, slice and serve.