In a large pot, bring 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 Romanesco, cover, reduce heat to medium, and steam for 2–3 minutes, until crisp-tender (add more water if needed). Using a slotted spoon, transfer Romanesco to a rimmed baking sheet or clean kitchen towel, spreading it in a single layer, to cool.

In a small bowl, whisk together mustard, lemon zest, lemon juice, and 1/4 teaspoon salt. Slowly add oil, whisking constantly to emulsify.

Put Romanesco in a serving bowl. Add bell pepper, onion, dill, capers, remaining 1/2 teaspoon salt, and vinaigrette; 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.