Preheat oven to 190 C. Heat the milk to almost boiling. Place the oats in a large bowl and add the hot milk. Allow to cool, stirring occasionally. As the mixture starts to cool, add the butter and mix through so that the butter melts. Meanwhile, measure out the plain flour, buckwheat flour, baking powder, sugar, salt and fennel seeds into a bowl. Mix to combine and then add this to the oat and milk mixture. Stir to form a dough. When the mixture comes together, turn the dough out onto a clean work surface and knead gently. You may need to add a little more flour to stop the dough sticking, but go easy on the flour. Try not to add too much as this will make the crackers dense.
Line some baking sheets with re-usable silicone liners or greaseproof paper. You can also lightly grease the baking sheets with a little olive oil to stop them sticking.
Using one quarter at a time, roll the dough out as thinly as you can. Add a bit more flour in between rolling to ensure it doesn't stick to your work surface. The thinner the dough the better. Aim for paper thin if you can. Using a roller cutter or a sharp knife, trim off the edges and then cut out squares or rectangles. You can also use different shaped cutters. Place them on the prepared baking sheets and sprinkle with a little salt. You can also brush them with a little olive oil if you wish, but this isn't strictly necessary. The oil adds a bit of richness.
Bake in a hot oven for about 4 minutes until the crackers start to bubble a bit and you can see golden patches. Take the tray out and flip the crackers so that the other side is now exposed. Bake for a further 4- 5 minutes watching them carefully as they can burn quite easily especially if you have brushed them with oil. If your crackers are thicker, they may require further baking time. When baked the crackers should be very light and very snappy with a nice golden colour.
Leave to cool on a wire rack before placing in an air tight container.
To make Indian Style crackers, similar in flavour to the crispy cylinders you find in Bombay Mix, you can substitute gram flour (made from chick peas and available from Asian shops)- for the buckwheat flour. Substitute garam masala for the fennel seeds and a bit of chili powder for kick. Please note that you may need to add additional plain gluten free flour to achieve a workable dough that is pliable and soft, but doesn't stick to your work surface.