To prepare the frittata: Heat the oil in a 25cm heavy based, non-stick frying pan. Add the potatoes, onion and peppers. Then stir through the mustard seeds, cumin and chilli flakes and season with salt and freshly ground black pepper. Cover and cook over a low heat for 10 to 15 minutes until the potatoes are soft but not brown. Stir occasionally to prevent anything from sticking.
Meanwhile whisk the eggs in a large bowl. When the potato mixture is cooked, using a slotted spoon, add it to the eggs along with the coriander and garam masala. Mix well and add a little more seasoning.
Preheat the grill to medium.
To cook the frittata: You can use the same frying pan to cook the frittata. Heat it over a medium heat. If there is a little oil left from cooking the potato mixture you can use that, otherwise add a little more oil. Pour the egg mixture into the pan. Move the pan around to let the egg mixture run in under the potatoes and get to the bottom of the pan.
Cook for about 10 minutes until nearly set. Place under the preheated grill and cook for about another 10 minutes until the top is set and golden.
To make the salad: Meanwhile make salad. Combine the scallions, tomatoes, coriander, chilli, garlic, oil, lime juice, garam masala and season with salt and freshly ground black pepper. Mix well then and over nigella seeds, if using.
To serve: Cut the frittata into four wedges and serve along with the Tomato and Coriander Salad and some crusty bread.