How to Make It
Bring 1 inch of water to a boil over medium-high in a large saucepan fitted with a steamer basket. Place cauliflower florets in steamer basket; cover and cook until tender, 8 to 9 minutes. Transfer cauliflower to a bowl of ice water. Let stand 5 minutes; drain well.
Whisk together vinegar, mustard, pepper, and salt in a small bowl. Gradually add oil, whisking constantly, until emulsified. Set aside.
Bring 1 inch of water to a boil over medium-high in saucepan fitted with steamer basket. Place eggs in steamer basket; cover and cook 8 minutes. Transfer eggs to a bowl of ice water. Let stand until chilled, about 6 minutes. Drain. Peel eggs, and cut in half lengthwise.
Combine cauliflower, cornichons, dill, parsley, and chives in a large bowl. Drizzle with vinegar mixture, and toss to coat. Top with egg halves. Serve at room temperature.