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1
Place mustard seeds in a small heatproof jar or bowl. Bring vinegar, sugar, and salt to a simmer in a small saucepan over medium-high heat, stirring to dissolve sugar and salt. Pour brine over mustard seeds and let cool, about 2 hours.
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2
Bring shallot and oil to a simmer in a small saucepan over medium heat. Cook 1 minute (watch it carefully; you don't want the shallot to take on any color). Let cool, then pour oil through a fine-mesh sieve into a small bowl; discard shallot.
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3
Whisk shallot oil, vinegar, soy sauce, and sugar in a small bowl to combine; season vinaigrette with salt.
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4
Trim ends from cucumbers and very thinly slice lengthwise on a mandoline or with a vegetable peeler or sharp knife into long ribbons. Toss cucumbers, arugula, celery, celery leaves, and dates in a large bowl to combine.
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5
Drizzle half of dressing over slaw and add 2 Tbsp. drained pickled mustard seeds (save remaining mustard seeds for another use) and toss again to coat. Taste and season with salt.
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6
Transfer slaw to a platter and top with sesame seeds. Serve with remaining vinaigrette alongside.
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7
Mustard seeds can be pickled 1 week ahead. Cover and chill.
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8
Vinaigrette can be made 2 days ahead. Cover and chill. Bring to room temperature before using.
