Whisk and season the eggs. Halve the buns. Peel and coarsely grate the onion, then put 1 tablespoon into a bowl with a pinch of sea salt and a splash of red wine vinegar, and leave to quickly pickle. Scatter the rest into a large non-stick frying pan on a medium heat with 1 tablespoon of olive oil and most of the garam masala. Fry for 3 minutes, stirring often, while you coarsely grate the tomatoes. Push the onions aside and pour in the tomatoes to sizzle for 2 minutes. As soon as they start to dry out, mix and fry for 2 more minutes, or until thick, stirring occasionally.
Push the tomato chutney to one side, then toast the buns on both sides, wiping them around the pan to pick up any gnarly sticky bits. Spread the tomato chutney on the bun bases, tear off and pile on the coriander leaves, squash the lids on top, then remove to your plates. Scramble the eggs to your liking and spoon alongside, then top with the drained pickled onion and serve dusted with garam masala.