reheat oven to 350°F In a large bowl, combine all the dry ingredients (first 5 listed). In a small bowl, whisk the egg with the oil. Pour the liquid ingredients into the flour mixture and stir well. Add the cheddar and jack cheeses, stir to combine, then knead together lightly to form a ball of dough.
Scoop up the dough and place on a sheet of parchment paper. Cover with a second sheet of parchment paper; roll out the dough with a rolling pin to 1/8 inch thick, or less. You want it to be thin and even, but not too thin or they will break easily. Even up the edges best as possible with your hands, then roll out again lightly so the dough is “square-ish”.
Using a pizza wheel or knife, cut the dough into the size crackers you would like (1 -inch squares are nice). Sprinkle the dough with a little coarse sea salt; lightly press the salt into the dough. Transfer the crackers with the bottom sheet of parchment paper to a baking sheet.
Bake for around 15 minutes, or until the crackers are very lightly browned and crisp. If your oven has a convection fan, turn it on. Also it helps to turn the pan around half way through baking since the oven temperature may vary front-to-back. Keep an eye on them so they don’t dry out or burn!
Let them cool (if you have the patience), then serve. Store in a container with a sealed lid to maintain crispness.