Heat the oil in a saucepan and cook the cumin and mustard seeds until they start to pop.
Add the turmeric, chillis, onion, garlic and ginger, season with salt and cook until the onion is tender.
Add the tomatoes and chickpeas and simmer for 15 minutes.
Put the spinach in a heat-proof bowl and cover with boiling water. Turn to wilt, drain and squeeze out the excess water. Chop roughly and stir into the tomato mixture along with the tamarind concentrate. Cook for 2 minutes. Taste and season with salt if needed.
If the paneer has been packed in water or whey, pat dry with kitchen towels. Slice the paneer into 2 cm thick batons.
Heat ½ cm of vegetable oil in a sauté pan and cook the paneer until golden on both sides. Transfer to kitchen towels to drain then cut into large bite-sized pieces and combine with the tomato sauce.
Transfer to a shallow serving bowl to serve.