Preheat the oven to 375 degrees F. Line two sheet pans with parchment paper.
Place the oats, flour, brown sugar, and kosher salt in a food processor fitted with the steel blade and pulse for 45 to 60 seconds, until the oats are coarsely ground. Add the butter and pulse 15 to 20 times, until the butter is the size of peas.
Dissolve the baking soda in the lukewarm water and drizzle it over the crumbs in the food processor. Pulse until the mixture is evenly moistened and can be pressed into balls that will hold together.
With a medium (1 3/4-inch) ice cream scoop or two spoons, scoop the dough into your hands and roll them into round balls. Evenly space 12 balls on the prepared sheet pans. Flour the bottom of a flat-bottomed 3-inch-diameter drinking glass and use it to flatten each cracker to 1/8 inch thick and about 3 inches in diameter. The crackers will not be perfectly round. If there is a lot of flour on the crackers, turn them over with a metal spatula. Sprinkle with the fleur de sel and bake for 20 to 25 minutes, until golden brown on the edges. Serve warm or at room temperature.