Preheat the oven to 400°F.
Rub the beef with olive oil, garlic, salt, pepper, and dried herbs.
Place the beef on a roasting pan and roast for 35 to 45 minutes, or until it reaches your desired doneness.
Remove the beef from the oven and let it rest for at least 10 minutes before slicing thinly against the grain.
While the beef rests, prepare the dressing by whisking together sour cream, horseradish, Dijon mustard, lemon juice, olive oil, salt, and pepper.
Arrange the salad greens on a large serving platter or individual plates.
Top with tomatoes, cucumber, red onion, and slices of roast beef.
Drizzle with horseradish dressing just before serving.