Its color is fantastic in this lively salad, though you can definitely use white cauliflower if that’s all you can find. Cook the Romanesco just long enough to take away the raw bite, 2 to 3 minutes tops. Normally I would suggest plunging the florets into ice water to halt the cooking immediately, but introducing extra water here will mute the flavor and dilute the dressing. Instead, cook them fast and then spread them on a baking sheet so they cool quickly.
In a large pot, bring the water to a boil over high heat. (If you have a steamer insert, put it in the pot to hold the Romanesco. If you don’t have one, don’t worry about it.) Add the Romanesco, cover the pot, turn down the heat to medium, and steam for 2 to 3 minutes, until crisp-tender. Using a slotted spoon, transfer the Romanesco to a rimmed baking sheet or clean kitchen towel, spreading it in a single layer, to cool.
In a small bowl, to make the vinaigrette, whisk together the mustard, lemon zest, lemon juice, and 1/4 teaspoon of the salt. Slowly add the oil, whisking constantly with a fork to form an emulsified vinaigrette.
Put the Romanesco in a serving bowl. Add the bell pepper, onion, dill, capers, the remaining 1/2 teaspoon salt, and the vinaigrette and toss gently to combine. Cover and refrigerate until ready to serve. It will keep well for several hours. Just before serving, taste and add more salt if needed.
Reprinted with permission from Brassicas by Laura B. Russell (Ten Speed Press, © 2014). Photo credit: Sang An.