In a large heatproof bowl, whisk together rye flour, bread flour, salt, and yeast.
In a small pot over medium heat, warm the heavy cream and water until your instant-read thermometer reads between 110°F and 120°F. (If your dairy mixture gets too hot, remove the pot from heat and allow to cool to 110–120°F.)
Make a well in the middle of your dry ingredients and pour in liquids. Mix together with a wooden spoon to form a rough, sticky dough.
Tip dough out onto work surface dusted with rye flour. Knead by hand for 5–10 minutes until dough is smooth. NOTE: If you’ve kneaded for at least 5 minutes and your dough is still really sticky, allow dough to rest for 5–10 minutes, then continue kneading.
Divide dough into 6 equal pieces, forming each piece into a ball.
Cover dough balls with a light (and clean!) tea towel and place in a warm, dry spot and leave undisturbed until puffy, about 30 minutes.
Line baking sheets with parchment paper.
Towards the end of the dough’s rising time, heat oven to 475°F.
Keeping the remaining dough balls covered, place 1 dough ball on work surface dusted with rye flour. Punch it down to release excess gas build-up.
Using a rolling pin, roll dough into a 6-inch circle. Transfer to one of the prepared baking sheets.
Continue rolling until dough is quite thin and 10–12 inches in diameter. Sprinkle with 1 teaspoon dill weed, ½ teaspoon sea salt, and ½ teaspoon poppyseeds. Roll over toppings to help them stick to dough. (At this point, your dough should be roughly 12–14 inches in diameter.) NOTE: It’s okay if your knäckebröd isn’t a perfect circle. Ours weren’t, and they were still darn tasty!
Dock the dough aaallllllllllllll over with a fork. 
Bake for 5 minutes. Carefully flip crispbread with tongs, then rotate pan in oven. Bake for 3 minutes more, keeping a close eye, as crispbread can go from perfect to burnt rather quickly since they’re so thin.
Transfer to wire rack to cool completely.
Repeat with remaining 5 dough balls, making sure to allow baking sheets to cool between each bake. (The crispbread is incredibly thin, so the residual heat from the baking sheets will start to bake them immediately and may cause them to burn. Therefore, you must allow the baking sheets to cool.)
Once all crispbreads have been baked and oven has cooled down to 100–120°F, you can stack the crispbreads and place them in the oven to dry out even more. (We left ours in the oven overnight.)