Heat the oil a large skillet and add the paneer cubes, in 2 batches, and fry until they are golden. Remove the golden cubes to a double thickness of kitchen towel. (It is possible to dry fry the paneer cubes in the pan with no oil, to avoid the oil splashing you. Then continue with the recipe below.)
Pour all but about 2 tablespoons of the oil out of the pan. Put the onion, garlic cloves, and ginger into a food processor and blitz to a coarse pulp. Fry gently for about 5 minutes with a sprinkling of salt. Stir in the garam masala and turmeric and cook for another 2 minutes before adding the still frozen peas.
Dissolve the tomato puree in the vegetable stock and pour over the contents of the pan. Stir again and turn the heat down to low, cover with foil or a lid and cook for 15 minutes, tasting to check that the peas are tender. You can cook muttar paneer up to this stage, if you like, uncovering and then reheating gently with the diced, oil-crisped cheese, or proceed directly now.
In which case, take off the foil and add the paneer cubes to warm them through before serving.