Add 7-grain cereal and rolled oats to a large bowl, or the bowl of a stand mixer fitted with a kneading hook. Pour boiling water over. Stir, then let stand until water cools to just lukewarm, about 10 minutes.
*Be sure mixture is not too hot when you add the yeast. For instant yeast, you want it no more than 125F. For active dry yeast, it should be more like 110F.
Add yeast, wheat bran, brown sugar, ground flax seed, sunflower oil and salt. Stir to combine. Add mashed banana and walnuts and stir in. Add 1 1/2 cups of the flour and stir to combine. *If mixing by hand, remove dough from bowl to a very well floured surface if it gets too hard to stir and begin adding flour through kneading instead.
Continue adding flour 1/4 cup at a time at first, then a tablespoon at a time until you have dough that forms a ball, but that is quite moist and just a bit sticky. If making by hand, remove dough to a greased bowl. If making in a stand mixer, just leave the dough in the mixer. Cover bowl or stand mixer bowl with plastic wrap and let stand 45 minutes.
Preheat oven to 375F and have two baking sheets out and ready.
Remove dough to a well floured surface and gently deflate. *At this point, I use my kitchen scale to measure the weight of the entire dough ball in grams. I then divide that amount by 10 (since we will be needing 10 pieces) and I start cutting off and weighing pieces of dough, adjusting so that they are the calculated weight. If you don't have a kitchen scale, start by cutting the dough ball equally in half, then form each half into a bit of a log. Do your best to cut the log into 5 equal pieces. Repeat with the other half of the dough.
*As the dough is very moist, dust surface and top of dough with flour as often as needed to prevent the dough sticking to the surface or the rolling pin.
Working fairly quickly, form each piece of dough into a ball. Working with one ball of dough, roll into a 1/4-inch thick and about 5 1/2 inch diameter circle. Place on prepared baking sheet as you go, until you have rolled all 10 (placing 5 pieces on each baking sheet).
*You want to complete this process fairly quickly, to prevent the dough from rising too much further before they go in the oven. Otherwise, your rounds will end up too thick. For this reason, you will also need to bake them off in one batch (rather than one sheet at a time).
With floured fingertips, make dimples in the top of each dough round by pressing all your fingers down on the dough a couple of times (to deflate the dough a bit). Brush tops of rounds with milk or water (to keep them soft, rather than getting crispy), then immediately place both baking sheets in the oven. If your baking sheets won't fit side by side on one rack, place one on a rack positioned in the top third of the oven and one on a rack positioned in the bottom third of the oven.
*If you are baking your rounds on two racks in the upper and lower thirds of the oven, remove and flip the rounds after about 8 minutes, then return the rounds to the oven placing on the opposite racks and baking a further 5-7 minutes.
Bake in preheated oven for about 10 minutes or until the underside of the rounds are golden in spots. Remove trays from oven. Flip rounds over and bake another 3-5 minutes, or until the rounds are golden on the underside. Remove trays and immediately remove rounds to a cooling rack to cool completely.
Once cooled, store in an airtight container in the fridge for up to 1 week or freeze.