Beat the egg whites with an electric mixer. Add lemon juice and salt. Keep beating until you get a firm meringue. Add powdered sweetener and continue beating at a high speed. When your meringue is thick and firm, reduce the speed of your mixer to the minimum and add psyllium. Gently mix.
Add walnut meal spoon by spoon, and gently mix with a spatula.
Line 2 large baking pans (50x50 cm 20x20 inches) with a parchment paper and pour in 1/2 of the mixture in one pan and the other half in the other pan. Using a large knife even the mixture to get a thin layer of custard.
Place both pans into the oven at 100ºC (212ºF) for an hour. Since they will be at different altitudes, flip them so that the lower one is also exposed to the top heater, and bake for 45 minutes total. They should dry out, but not burn.
If your oven cannot have 2 pans at the same time, prepare the mixture with half ingredients. Bake the first custard, and then repeat everything with the second one. In this case, you'll need more time, but patience is a virtue!
When the custards are ready, cut them in half to get 4 rectangular shapes 25x50 cm (10x20 inches)