Step 1To 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 2Trim 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.