Heat half the olive oil in a frying pan, add the mushrooms and fry until they have coloured and they release some liquid.
Add the garlic and cook for a minute.
Meanwhile mix together the tahini, water and soy sauce. Add to the pan and allow to thicken slightly.
Add the lemon juice and parsley and season with salt (a little extra water if the sauce is still too thick). Set aside.
To fry the sage leaves, heat the remaining oil in a small saucepan over a medium heat. Add the sage leaves and cook gently until they start to turn translucent. Remove from the pan and drain on paper towels (they will crisp up as they cool).
Top the toast with the mushrooms and scatter over a few sage leaves and some of the lemon zest.