Peel and finely slice (with the help of a supervising adult) the red onions and put them in a bowl. Add the red wine vinegar, salt and sugar, and toss to combine, squeezing the onions firmly between your fingers before setting aside.
Step one - Pickle the onions
Step two - Make the spiced mushrooms
Cut the caps off the king oyster mushrooms and set aside for another recipe. Roughly rip the stems of the king oyster mushrooms longways to “shred” them (if using portobello mushrooms use the whole mushroom instead). Put the plant-based butter and vegetable oil in the frying pan and set it over a medium heat. Add the fajita spice mix and mushrooms to the hot pan, stir to coat, and fry for about 5 minutes until darkened
Step three - Chipotle mayo
Add the mayo and chipotle paste to a small bowl. Squeeze in the juice of the lime. Mix everything together, adding salt to taste.
Step four - Serve and enjoy
Warm the tortillas under the hot grill or in a dry frying pan (with grown-up help). Slice the lettuce and fill each taco with a handful. Top each taco with mushrooms, pink pickled onions, and chipotle mayo. Slice the lime into wedges and serve alongside