Preheat the oven to 350°F (180°C) and line a baking pan with baking paper.
In a large mixing bowl, combine the almond flour with the psyllium husk powder, baking powder and salt.
Add the hot water and vinegar and stir.
Whisk the egg whites lightly, and fold into the mixture.
Transfer on the lined baking pan and spread put with the back of a spoon.
Bake for about 40 minutes until golden brown.
With a knife cut the dough lengthwise in half. Place back on the baking pan, cut side facing up.
Turn the oven’s setting to grill and raise the temperature to 450°F (230°C).
Make the topping by whisking together the grated parmesan cheese with the butter. Brush the mixture on top of each piece of dough and return to the oven for 5 more minutes.
Allow to cool down and break into smaller, crouton-like pieces.