1. Separate the broccoli tops into small pieces. Peel the broccoli stems and cut them in half lengthwise and then into 1/4 inch pieces. Steam or boil the broccoli until tender, about 4-5 minutes. Drain and let cool.
2. Whisk together the vinegar, horseradish, mustard, salt, and pepper. Add the oil slowly as you continue whisking. Whisk in the sour cream.
3. Place broccoli, beef, tomatoes, and scallions in a bowl. Toss with the dressing to coat.