1. Separate the broccoli tops into small florets. Peel the broccoli stems and cut them in half lengthwise and then into 1/4-inch slices. In a large saucepan, steam or boil the broccoli until tender, 4 to 5 minutes. Drain and let cool.
2. In a large glass or stainless-steel bowl, whisk together the vinegar, horseradish, mustard, salt, and pepper. Add the oil slowly, whisking. Whisk in the sour cream. Add the broccoli, beef, tomatoes, and scallions and toss to coat.
Wine Recommendation: White wine may seem an odd choice for roast beef, but the dominant elements here are horseradish and broccoli, not meat. Accordingly, pull the cork on a California sauvignon blanc for an exciting pairing.