- Preheat the oven to 200 degree C. Wash the button mushrooms and new potatoes. Halve the potatoes. In a pan, bring the oil to heat on high. When it’s hot, sizzle the cumin seeds and stir through the ginger and garlic. Then toss in the mushrooms and potatoes, the salt, turmeric, chilli and coriander powders. Mix the lot together for about a minute and stir through the tinned tomatoes. Stick the pan in the oven uncovered for 20 minutes, stirring once half way, until a fork can be inserted into the potatoes easily. Take the pan out carefully, mix in the garam masala and sprinkle the fresh coriander on the mushroom potato to serve.