First, prepare the jelly. Place the mineral water with stevia and ground vanilla in a deep saucepan and bring it to a boil. Add beef gelatine mixed with 4 tbsp of cold water to the boiling mineral water mixture and stir well. Let it boil for 1 minute and remove from heat. Set aside to cool down.
Beat 1 whole egg, 6 egg yolks, salt, stevia, nutmeg and lemon juice (or citric acid) with an electric mixer.
Melt the butter and add it to this mixture. Keep beating until you get a shiny emulsion. Now add mascarpone and beat for another 3-4 minutes.
Add psyllium powder and almond flour while still beating at a high speed. In the end, sprinkle with baking soda and reduce the speed of your mixer.
Grease a 13x9" (33x23 cm) pan with some butter and pour in the batter. Bake in the oven for 20 minutes at 160ºC (320ºF)
Now that your Jelly cooled down halfway, add the blueberries and crush them a bit with a fork so that they release some of their juice and give the colour to the jelly. Place this mixture in the refrigerator.
Beat the egg whites with lemon juice and salt until you get a firm meringue. Add stevia and keep beating. When your meringue is firm, add crushed walnuts and mix gently with a spatula.
Take your pie base out of the oven and reduce oven's temperature to the lowest point possible. Somewhere between 50º and 70ºC. (122-158ºF)
Spread the blueberry jelly all over the baked pie base. On top spread meringue with walnuts. Return to the oven and let it dry, rather than bake, for an hour without opening the oven. In the end, for just 3-4 minutes, increase the temperature to get a nice golden crust on top.