Preheat oven to 325F/ 162C and line a baking sheet with parchment paper.
In a medium bowl, mix all the ingredients together until combined. Then using a rubber spatula or offset spatula spread the batter out onto the prepared baking sheet. It will look a bit like cake batter. Spread them thin but not so thin you can see the paper. Sprinkle with a little more sea salt if desired.
Place in the preheated oven and bake for 10 minutes. Then remove and score with a knife into squares or however you like. You can also opt not to score and break into pieces later, but I like to be able to remove the outer crackers when they turn brown before the ones in the middle. (*I find that pushing the knife down works better than dragging or sliding it to score them.)
Place back in the oven and bake for another 35-45 minutes. Check them every 15 minutes or so and rotate the pan. You want them just slightly golden brown. When you remove one from the oven and let it cool a bit it should be crispy and snap rather than bend.
Remove from the oven and let cool on the sheet for about 10 minutes then transfer to a cooling rack. They should snap apart right where you scored them. Once cool, place in an airtight container or jar. They will keep for a week.
*You can use other seeds besides the flax and poppy seeds or opt to use no seeds at all, but they add nice texture and a little dose of nutrition.
*You can omit the rosemary or trade for another herb.
*You can swap the butter for olive oil, but I really do recommend the butter here. The flavor and texture difference is far better to me.
*There's a fine line between not baked enough resulting in crackers that bend and being over-baked, so watch them in those final moments in the oven, testing one on a cool counter as soon as it turns a very light golden brown.