Blanch the spinach in a pan of boiling water for 3 to 4 minutes, or until wilted. Drain in a colander, squeeze out any excess water, then blitz in a food processor until roughly pureed. Set aside.
Heat 1 tablespoon of oil in a wide non-stick frying pan.
Stir-fry the paneer for 2 to 3 minutes, or until lightly golden. Remove with a slotted spoon and set aside.
Heat remaining oil in the same frying pan. Add the black mustard seeds & wait until it pops.
Next add cumin seeds and stir-fry for about 30 seconds & then add the onion and fry over a low heat for about 5 to 6 minutes, or until softened.
Add the grated ginger, garlic and garam masala to the pan and continue to cook for a further minute or so.
Then, stir in the spinach and cook over a medium heat for another 3 to 4 minutes.
Add the paneer and cream. Cook, stirring, for 2 to 3 minutes, or until piping hot. Consistency should be thick.
Squeeze in half the lemon juice, then taste and season, adding more lemon juice, if you like. Put to one side and keep warm.