Preheat the oven to 180 degrees Celsius (350 degrees Fahrenheit).
Place the walnuts in a food processor and pulse until you get a coarse powder.
Transfer to a bowl and set aside.
Heat up the oil in a frying pan and add the onion and garlic once hot.
Fry for 5 minutes until softened.
Add the mushrooms and fry for another 10 minutes until all the water from the mushrooms has been released and evaporated - don’t put a lid on otherwise the water won’t evaporate. You need to make sure all the water has gone from the mushrooms otherwise your meatloaf will be soggy.
Add the cooked onion, garlic and mushrooms to the same food processor that you used for the walnuts (no need to wash it out) with all the other ingredients.
Pulse until all ingredients are combined and you get a mixture that sticks together but still has some texture.
Transfer this mixture to the bowl with the ground walnuts and mix well.
Transfer to a loaf tin lined with greased baking paper.
Bake in the oven for 45 minutes, until firm to the touch and top is slightly cracked.
Leave to cool slightly before removing from the tin and slicing.
Leftovers keep covered in the fridge for up to a few days - reheat slices in a dry frying pan or in the oven.