Peel only toughest outer leaves from broccoli romanesco, leaving tender inner ones attached. Steam broccoli in a steamer basket over simmering water, covered, until tender to the core when pierced with a knife, 15 to 20 minutes. Put in a serving dish.
Meanwhile, mix remaining ingredients, seasoning with salt and pepper to taste.
Spoon about half the green herb sauce over broccoli and turn gently to coat. Serve warm or at room temperature, with extra sauce on the side.
*Find at farmers' markets in fall and winter, and increasingly at well-stocked grocery stores.
Make ahead: Sauce, up to 1 day, chilled (add lemon juice at the last minute, or it will dull the sauce's color). Broccoli romanesco may be steamed a few hours ahead; let sit, loosely covered, at room temp (if chilled overnight in a container, it gets sulfurous).
Note: Nutritional analysis is per serving with 1 Tbsp. extra sauce.