Roast 150g of walnuts and 80g of almonds in a dry pan over medium temperature. let them cool down totally and then, finely grind them.
Beat the eggs with sea salt, citric acid and sweetener. Add melted (but cooled) butter and beat for at least 5 minutes at high speed.
Add nutmeg, vanilla extract and psyllium. Beat for a minute. Add 4 tbsp of nuts and beat for another minute. Repeat adding 4 tablespoons of nuts and beating 2 more times until you use 12 tbsp of roasted nuts mixture.
Now sprinkle baking soda and mix. In the end, add sparkling mineral water while beating at a lower speed.
Line an 11-inch cake mould with parchment paper and grease the sides with some coconut oil or butter. Pour in the cake batter and bake for 35 minutes at 150ºC (350ºF) When it's baked, remove it from the mould and let it cool down totally!