First let's pickle the red onion, peel and finely slice the red onion into long thin slices and add to a glass along with the salt and vinegar, ensuring all the onion is submerged in the vinegar.
Mix together all the umami ketchup ingredients and mix well, taste and adjust the seasoning and spiciness to your liking. Take the oyster mushrooms and rip the larger ones in half lengthwise and add to a bowl along with ¾ of the umami ketchup and mix well, and leave to marinade for 5 mins while you make the tabbouleh and tahini cream.
For the tabbouleh quarter the cherry tomatoes, finely dice the cucumber, lettuce and coriander and add them all to a bowl, finely dice the chilli, include the seeds if you prefer it spicy or discard if too spicy and add to the bowl. Add a generous pinch of salt and mix well, taste and adjust the seasoning to your liking.
For the tahini cream, mix together all the ingredients with a fork until it comes together into a thick creamy delicious looking sauce, not that unlike mayonnaise. Taste and adjust the seasoning to your liking. If its too thick add a little more yogurt and if it's too thin add a little more tahini, you want it like mayo like consistency.
Carefully thread the mushrooms onto 2 skewers, heat a frying pan or griddle pan on high heat and add 1.5 tbsp of oil. Once hot, reduce to a medium heat, add the mushroom skewers and cook on all sides until they start to char. Use a pastry brush to brush on more of the sauce to ensure an even coating where the sauce almost glazes the outside of the mushrooms.Once the mushrooms are lightly charred all over remove from the heat.
To serve, heat up the flat breads and add a generous dollop of tahini cream on the base, followed by lots of tabbouleh, remove the mushrooms from the skewer on top and finely add some pickled red onion on top - enjoy!