In a large saucepan over medium-low, toast the bay, coriander, caraway, mustard seeds, peppercorns and pepper flakes, stirring, until fragrant, about 1 minute. Add 2 quarts water, the sugar and 3 tablespoons salt; bring to a boil over medium-high. Carefully add the cauliflower, stem side down, then cover and turn off the heat. The cauliflower will float; it’s fine if the top of the head is not submerged. Let cool until barely warm, then refrigerate for at least 12 hours or up to 24 hours.

When you are ready to cook the cauliflower, heat the oven to 425°F with a rack in the middle position. Remove the cauliflower from the brine and thoroughly pat it dry. Place it stem side down on a rimmed baking sheet. Roast until lightly browned and a skewer inserted all the way through the head and into the core meets some resistance, 40 to 45 minutes.

Meanwhile, in a small bowl, stir together the butter, mustard and 1 tablespoon dill. In another small bowl, combine the panko, oil and remaining 1 tablespoon dill; stir until the breadcrumbs are evenly moistened.

When the cauliflower is done, remove the baking sheet from the oven. Brush the butter mixture evenly over the entire head, then apply the panko mixture in an even layer, lightly pressing so it adheres. Return to the oven and roast until the coating is golden brown, 10 to 15 minutes. Using a wide metal spatula, transfer the cauliflower to a cutting board. Cut the head into wedges for serving.