Set a large saucepan over a medium heat then add in the veg oil & butter. Once the butter has melted, add in the sliced onions & a good pinch of salt then cook, stirring frequently until soft & lightly browned. This will take around 15 minutes.
Stir in the garlic, ginger, cashews, garam masala, paprika, cumin, turmeric, fenugreek, cinnamon & chilli powder. Cook this out for a minute, stirring frequently.
Next, stir in the tomato paste, cook out for another minute then add in the chopped tomatoes & the water (use this to rinse out the empty tomato tin).
Reduce the heat to low & cook the curry sauce for 20-30 minutes, stirring regularly until it has thickened & the spices have cooked out.You'll know when the spices have cooked out when the sauce doesn't taste gritty. If your sauce gets too thick, thin it out with a splash of water. The tomatoes should have reduced by roughly half. Next, transfer the sauce into a jug blender then blend briefly to break up the onions & cashews. Add in the double cream & lemon juice then continue blending until the sauce is smooth. Season to taste with salt then pour the sauce back into the pan.A stick blender will also work! Set the pan of sauce back over a low heat then once hot, stir in the butter a small piece at a time. Make sure to let each piece of butter melt completely before adding more.
Add the shredded turkey into the sauce then cover the pan with a lid & cook for another 5-10 minutes, until the meat is piping hot. Make sure to stir the curry regularly.
To serve, top the curry with a drizzle of double cream & a sprinkle of chopped coriander.