I do a variant on the Fandabi Bannock recipe on YouTube - he designed it to blend the qualities of pemmican, bannock and hard tack, and after a couple of practice runs mine taste gorgeous. You need to eat 2 to sustain you for the whole day similar to Lembas, and I def think they are filling.
The main ingredients are a flour, oats, and then fat (I use coconut oil but you can use a meaty one), some boiling water to hold it together, spices and flavourings, then bake forever in the oven.
I add chia seeds so you don't poop weird (source of fibre) , plus raisins and dark chocolate on top, but you can also make them savoury.