Step 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.

Step 2

Do Ahead: Mustard seeds can be pickled 1 week ahead. Cover and chill.

Step 3

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.

Step 4

Whisk shallot oil, vinegar, soy sauce, and sugar in a small bowl to combine; season vinaigrette with salt.

Step 5

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.

Step 6

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.

Step 7

Transfer slaw to a platter and top with sesame seeds. Serve with remaining vinaigrette alongside.

Step 8

Do Ahead: Vinaigrette can be made 2 days ahead. Cover and chill. Bring to room temperature before using.