How to Make It
In a large heatproof bowl, combine the carrots and fennel. In a medium saucepan, combine 2 cups of water with the vinegar, garlic, mustard seeds, sugar, the 1/4 cup of salt and half of the jalapeño. Warm over low heat, stirring to dissolve the sugar. Pour the brine over the carrots and fennel and let stand for 30 minutes. Drain the carrots and fennel; reserve the brine for another use.
Preheat the oven to 450°. Arrange the carrots and fennel on a baking sheet. Drizzle with the olive oil and roast for about 30 minutes, stirring occasionally, until tender and caramelized. Transfer the vegetables to a large bowl. Finely chop the remaining jalapeño and add to the bowl, along with the bell peppers, chopped celery and olives. Season with salt, mix well and transfer to a platter. Garnish with celery leaves and serve.