Mix together the sourdough starter, flours, salt, and butter until you have a cohesive dough. It’s OK if it’s a little sticky. Turn the dough out onto a lightly floured work surface, divide into two portions, pat each into a rectangle, wrap in plastic wrap (or place in an airtight vessel), and refrigerate for 30 minutes, or longer — I’ve done this 24 hours in advance — until the dough is firm.

Preheat the oven to 350°F.

Working with one piece of dough at a time, unwrap the plastic wrap and reserve it. Very lightly flour a piece of parchment, a rolling pin, and the top of the dough.

Place the dough onto the floured parchment paper, and place the sheet of plastic wrap on top of the dough. Roll the dough as thinly as possible (see notes above), using your rolling pin to disperse it as evenly as possible over the parchment paper. It’s OK if the edges are ragged, but do try to make the dough as thin as possible — if it’s too thick, the crackers won’t be crisp.

Transfer the dough-topped parchment sheet onto a baking sheet. Lightly brush with oil and then sprinkle the salt (and/or other toppings) over the top of the dough.

Cut the dough as you wish. I like to do long strips, about 1.25 to 1.5 inches wide. I use my bench scraper, but a pizza wheel works well here, too.

Bake the crackers for 20 to 25 minutes, rotating the pan halfway through, until they’re starting to brown around the edges. Depending on how thinly you’ve rolled them, they may be done closer to 25 minutes, so do check at the 20-minute mark.

When fully browned, remove the crackers from the oven and place the pan on a rack to cool. I like to let the crackers cool completely on the sheet pan, which ensures they will be crisp ultimately.

Repeat with the remaining portion of dough or keep it in the fridge or freezer until you are ready to use it.

Store crackers in an airtight vessel at room temperature for up to a week. Freeze for longer storage.