To prepare the frittata: Heat the oil in a 25cm heavy-based, non-stick frying pan over a medium heat. Add the potatoes, onion and red peppers, then stir in the mustard seeds, cumin seeds and chilli flakes. Season with salt and freshly ground black pepper.
Cover and cook on low heat for 10 to 15 minutes. Stir now and then. Cook until the potatoes are tender and the vegetables are soft, not browned.
Meanwhile, whisk the eggs in a large bowl. Once the potato mixture is cooked, transfer it to the eggs using a slotted spoon. Add the coriander and garam masala, then mix until evenly combined.
Preheat the grill to medium.
To cook the frittata: Return the frying pan to a medium heat. If needed, add a little extra oil to lightly grease the pan.
Pour in the egg mixture, gently tilting the pan to ensure the eggs distribute evenly around the vegetables. Cook for approximately 10 minutes, or until the base is set and the surface is almost firm.
Transfer the pan under the preheated grill and cook for a further 8–10 minutes, until the frittata is completely set and lightly golden on top.
To make the salad: While the frittata is cooking, combine the scallions, tomatoes, coriander, chilli and garlic in a large bowl. Add the oil, lime juice and garam masala, then season with salt and freshly ground black pepper. Toss everything together until well combined and finish with the nigella seeds, if using.
To serve: Slice the frittata into four wedges and serve warm with the tomato and coriander salad and some crusty bread.