Heat the oil in a frying pan or saucepan over low-medium heat. Add the onions, cumin seeds, and a pinch of salt. Cook for 15 minutes, stirring occasionally, until the onions are browned on the edges.
Add the garlic, ginger, cumin powder (if not using seeds), and a modest amount of salt. Fry for a minute, stirring frequently.
Add the curry powder, and optionally the chilli powder and fenugreek. Stir frequently for another minute.
Add the fresh chillies and tomato purée with a mug of water or stock. If using passata or tinned tomatoes, use half a mug of water/stock instead. Turn the heat to medium-high, mix well, and cook for 20 minutes, stirring occasionally.
Squeeze in the lemon juice, add the protein and vegetables, mix, and let warm through for a couple of minutes.
Add more water or stock to thin the sauce if desired, then taste and add more salt if needed. Serve topped with fresh coriander and a sprinkle of garam masala if desired.