Fry the mustard, coriander, cumin and fenugreek seeds for 2–3 minutes in a dry pan.
Transfer the fried spices to a pestle and mortar and grind with the curry leaves. When ground, add the other spices.
Add the oil and vinegar to the spices and mix up into a paste.
Lightly fry the mushrooms in a little oil until just brown.
Add 1 heaped tbsp of the spice paste and the water. (Cover the remaining spice paste and leave in the fridge to use another day.)
Place a lid over the pan and leave to heat for 2–3 minutes.
Add the red onion and plum tomato and allow to cook for 1–2 minutes.
Add the spinach and stir into the hot bhaji.
Season to taste with salt and pepper and serve sprinkled with coriander and drizzled with sour cream with Naan bread on the side.
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