Thinly slice the garlic and cut the mushrooms into bite-sized pieces.
Heat the oil in a pan and fry the garlic for 30 second over medium heat, then add the mustard seeds and cumin seeds.
Stir-fry for 10 seconds, then add the other spices and tomato purée. Stir for a minute, then add the mushrooms and a drop of water. Season with salt & pepper.
Put a lid on and cook for about 8 minutes. The mushrooms should have released their water by now. If the sauce is too sticky, add some more water - if there is too much liquid, reduce it by turning up the heat. Cook for another 5 minutes or so, until you get a very thick sauce.
Add some lemon or lime juice, then serve, garnished with fresh herbs.