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1
In a medium mixing bowl, whisk together flour and salt. Add water a little at a time, whisking to break up any lumps, until you have a smooth, thin batter. You may end up using a little more or less water. Cover the bowl with a towel and let sit at room temperature for at least 30 minutes and up to 12 hours-the longer, the better. (The batter may separate as it sits, which is fine.)
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2
While the batter sits, prep the asparagus: cut off the floret tips and cut each floret in half. Use a vegetable peeler to shave the asparagus stems into thin strips (I found it easiest to lay the asparagus flat at the edge of a cutting board and hold onto the untrimmed end). Toss the shaved asparagus and the leek slices with a olive oil, salt, and pepper, and set aside.
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3
Preheat the oven to 450u00b0 F. Heat olive oil in a large, well-seasoned cast iron skillet (or other oven-safe skillet) on a burner over medium-high heat, until the oil is hot but not quite smoking. Whisk the batter to bring it back together if it's separated. Pour the batter into the skillet, transfer the skillet to the oven, and bake for 25-35 minutes, or until the flatbread is firm and just starting to brown around the edges.
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4
Remove the skillet from the oven and turn on the broiler. If there are any bubbles in the flatbread, prick them with a knife and flatten them out. Spread the mustard on the flatbread, then distribute the asparagus and leeks evenly over the top. Return the skillet to the top rack of the oven and broil for 3-5 minutes, or until the asparagus and leek slices are lightly charred.
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5
Remove from the oven and let sit for a few minutes before slicing into wedges. Serve warm or at room temperature.
