Pour 1/2 inch of water into a pan fitted with a steamer basket. Place the broccoli in the basket. Cover, and bring to a boil. Cook until tender, about 4 to 5 minutes. Drain, and cool.

To make the dressing, whisk together the vinegar, mustard, horseradish, salt, and pepper in a bowl. Continue whisking while slowly adding the olive oil to make a smooth mixture. Whisk in the sour cream.

Place the broccoli, roast beef, tomatoes, and green onions into a bowl. Stir in the dressing and toss until evenly coated. Divide the salad evenly among 4 serving plates.