1. Preheat the oven to 400°F.
2. Chop the onion and grate the potatoes coarsely. Sauté in an ovenproof frying pan and reduce to medium heat.
3. Beat eggs with salt and pepper. Pour mixture over the onions and potatoes in the pan. When the bottom has set and the top is almost set, top with small pieces of Castello Double Crème Blue.
4. Put the pan in the oven and bake for approximately 10 minutes, or until the top is golden brown and the cheese has melted. Top the frittata with a handful of chopped walnuts, sliced radishes and chives.