While the paneer is cooling, prepare the spinach curry. Rinse the spinach leaves well and remove the veins (stems) if you are using large spinach leaves.
Put the spinach into a pot of boiling water. You can add the cashew nuts which will bring smoothness to the dish.
Cook for 3 minutes, drain and rinse with cold water to cool the spinach. Set aside.
Finely chop the onion, garlic and ginger.
Fry the onion in butter or oil for a few minutes until it becomes translucent.
Add the garlic and ginger and sauté for another minute.
Then add the tomato, then the spices, salt and pepper (adjust the seasoning to your taste).
Simmer over low heat so that the water in the tomatoes evaporates, stirring from time to time.
Meanwhile, in a blender, mix the spinach with a dash of lemon juice and/or water. If you have difficulty obtaining a purée, mix with a spoon in the blender bowl and remix with the blender on. You can also simply chop the spinach finely if you prefer a less liquid texture.
Add the spinach puree to the tomato/spice mixture and mix well to homogenize. Simmer for 5 to 10 minutes over very low heat.
In a frying pan, fry the cubes of paneer in a drizzle of oil or a knob of butter. Brown on one side before turning the cubes over to brown the opposite side. You can also skip this step and add the paneer cubes directly without browning them.
Incorporate the paneer cubes to the curry, or arrange the paneer cubes when serving on top of the spinach curry.
Serve with rice, green lentils or naans or simply enjoy on its own!