Soak the rye berries in boiling water. This will need to sit for at least two hours, and possibly longer; the rye berries should be soft enough to break apart if you press between your fingernails!
Once the rye berries have softened, add cracked wheat/bulghur wheat, sunflower seeds, and flax seeds to the same bowl as the rye berries and water. Mix together.
In a large mixing bowl, add warm water, yeast, Greek yoghurt, and honey. Mix together and let the yeast bloom for about 10 minutes - it should be creating bubbles on the surface (if you're using instant yeast, you can skip this step and add these ingredients directly to the seed mixture).
Add in the rye flour, bread flour, and salt, and mix together until the dough begins to clump together. Add in all the seeds and stir until evenly distributed.
Cover the dough with cling film or a tea towel and let it rise for 30 minutes.
After 30 minutes, wet your hands and knead the dough gently together with your hands for about 1 minute. It will be extremely sticky, but try to lift one edge and press towards the opposite side, repeating that move for about a minute.
Let rest for another 30 minutes; then repeat the same kneading step for 1 minute.
Rest for an additional 30 minutes; repeat the same kneading step for 1 minute (you will complete three total kneading sessions).
Cover and let rise for another 30 minutes.
Shape the dough by flipping it out onto a well-floured surface. Dust the top with flour, and shape the dough into a square shape, about 3/4 inch (2cm) thick. Cut the dough square into 9 rectangles (you can use a bench scraper or a knife).
Transfer the rolls to a baking sheet, cover with cling film or a tea towel, and let rise for 30 minutes.
While they are resting, place a deep baking dish/roasting tray on the bottom rack of the oven and preheat the oven to 390F (200C).
Towards the end of the rising time, boil a kettle with about 200-300ml of water.
When the rolls are done rising, brush with water and sprinkle additional sunflower seeds/flax seeds on top of the rolls (optional!)
Place the rolls in the oven and carefully pour the boiling water from the kettle into the roasting dish/baking tray on the bottom rack. This will create a lot of steam, so be careful!
Bake for 20-25 minutes until the rolls develop a golden color; it will be hard to tell how done the rolls are due to the dense dough, so you can always check with a thermometer. If the inside of the roll is at least 203F (95C), the rolls should be baked through.
Remove from the oven and let cool on a cooling rack. Enjoy with savory or sweet toppings - either will be delicious!