1. Place potatoes and water in a microwave safe bowl or microwave steam bag. Microwave on high for 8 to 10 minutes, until just tender, then let rest 10 minutes. Run cold water over the potatoes to stop further cooking.
2. While potatoes cook, prepare the dressing. In a small bowl, whisk the vinegar, salt, pepper, and mustard until evenly blended. Whisk in the oil until emulsified, then gently stir in the blue cheese, so the crumbles keep their shape.
3. Place the onion in a large serving bowl. Add a layer of potatoes, then roast beef and tomatoes. Drizzle the dressing over the salad. Top with a layer of romaine.
4. Refrigerate until needed. Toss well before serving.
Source: Hannaford fresh Magazine, March - April 2011