Ingredients

• 1/4 cup brown mustard seeds
• 1/3 cup unseasoned rice vinegar
• 2 tablespoons sugar
• 1/2 teaspoon kosher salt
• 1 small shallot, thinly sliced
• 1/3 cup olive oil
• 1/4 cup unseasoned rice vinegar
• 2 tablespoons soy sauce
• 1 teaspoon sugar
• Kosher salt
• 2 Persian cucumbers
• 1 bunch mature arugula, tough stems trimmed (about 8 cups)
• 4 celery stalks, thinly sliced on a diagonal, plus 1 cup celery leaves
• 10 Medjool dates, sliced
• Toasted sesame seeds (for serving)

Instructions

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.
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.
3. Whisk shallot oil, vinegar, soy sauce, and sugar in a small bowl to combine; season vinaigrette with salt.
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.
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.
6. Transfer slaw to a platter and top with sesame seeds. Serve with remaining vinaigrette alongside.
7. Mustard seeds can be pickled 1 week ahead. Cover and chill. Vinaigrette can be made 2 days ahead. Cover and chill. Bring to room temperature before using.