Step 1 To make the mustard-caper butter, pound the garlic with a half-teaspoon salt in a mortar until smooth. Stir the garlic into the butter with the mustard, capers, lemon zest and marjoram. Season to taste with pepper. (The butter can be made a day ahead and refrigerated. Bring to room temperature before serving.)
Step 2 Trim the base off the Brussels sprouts, then slice them in half or, if large, into quarters. Cut the cauliflower into bite-sized pieces.
Step 3Bring a large pot of water to a boil and add salt. Add the Brussels sprouts and cook for 3 minutes. Then add the other vegetables and continue to cook until tender, about 5 minutes. Drain, shake off any excess water, then toss with the mustard-caper butter. Taste for salt, season with pepper and toss again.