In a large wok or non-stick skillet, melt the butter over medium-high heat. Add the quartered mushrooms and cook for about 10 minutes until they release their moisture and start to brown. Then, transfer the mushrooms to a bowl and set aside.
Using the same pan, heat the sunflower oil over medium-high heat. Add the cumin and fennel seeds, stirring constantly for about 30 seconds until they give off a nutty aroma. Next, add the chopped onion and cook for 12-15 minutes until it turns golden. Lower the heat to medium and mix in the chopped garlic, frying for another minute.
Stir in the ground ginger, turmeric, Kashmiri chilli powder, and garam masala. Then add the can of chopped tomatoes and a teaspoon of caster sugar. Let the mixture cook uncovered for 5-7 minutes until it thickens and a thin layer of oil forms along the edges of the pan.
Spoon the Greek yoghurt into a small bowl and mix it with a ladleful of the hot masala to temper it. Then, stir this mixture back into the pan.
Pour in 100ml of hot water and allow the curry to simmer for 3-4 minutes until it reaches a creamy, double-cream-like consistency. Season it with salt and pepper to taste.
Return the cooked mushrooms to the pan and stir them in until they’re heated through. Finally, scatter the chopped coriander over the top with a little extra greek yoghurt and serve the curry with boiled rice or naan as you prefer.