Heat 1 Tbsp of the oil in a large, wide saucepan and add the paneer. Fry for a couple of minutes until it is golden, then transfer it to a bowl and set aside.
Add the remaining 2 Tbsp oil to the same pan over a low heat. Add the mustard and cumin seeds and fry gently for about 30 seconds until they become fragrant. Add the onion and continue to cook for about 5 minutes until softened.
Peel and finely grate the ginger and garlic, then add it to the pan along with the curry powder. Cook for another minute, then add the spinach and cook for a couple more minutes.