Thinly slice half the onion; soak in cold water. Mince 1 tablespoon of the remaining onion half; process with the goat cheese, buttermilk, vinegar, olive oil, horseradish, mustard, 1/4 teaspoon salt, and pepper to taste in a blender until smooth. Add 1 tablespoon water if needed.
Put the potatoes in a pot of salted water; bring to a simmer and cook until tender, about 18 minutes. Drain and pat dry. Meanwhile, put the eggs in another pot of salted water; boil for 8 to 10 minutes, then remove to a bowl of cold water. Add the green beans to the same pot of boiling water; cook until crisp-tender, about 3 minutes. Drain, run under cold water and pat dry. Peel and slice the eggs.
Drain the red onion slices. Toss in a bowl with the salad greens, green beans and half of the dressing. Top with the potatoes, tomato and hard-boiled egg slices. Drizzle with the remaining dressing.
Per serving: Calories 300; Fat 13.5 g (Sat. 5 g; Mono. 5.5 g; Poly. 1.4 g); Cholesterol 222 mg; Sodium 260 mg; Carbohydrate 31 g; Fiber 6 g; Protein 16 g Photography by Antonis Achilleos.