Add the onion, garlic, chilli, ginger, mint and cilanto (coriander) to a small food processor and pulse until finely chopped. Alternatively you can finely chop all of the ingredients and mix them in a bowl.
In a large shallow casserole dish or skillet, heat a little olive oil and toast the cumin seeds and mustard seeds until fragrant. Add the onion mix from the blender and saute until the raw onion smells starts to dissipate.
In a seperate frying pan, heat some olive oil on high heat and pan fry the mushrooms in batches until they just start to reduce. Transfer them into the pan with the spices and onion mixture (keep that pan on a low heat throughout if you can manage the two pans simultaneously, otherwise keep the heat in that pan off until they've all been added).
Once all of the mushrooms are added, stir the tumeric and garam masala through and add the canned tomatoes.
Simmer for 15 minutes while you prepare your rice, or longer if you have the time.
