Combine the marinade ingredients together and set aside while you prepare the mushrooms. You can either whisk the marinade ingredients together or give them a quick pulse in blender.
Clean and trim the mushrooms, cutting the larger ones into quarters and the smaller ones in half.
Add them to a bowl with the fennel and Spanish onion and pour over the marinade. Stir well to combine and then transfer to the fridge** to "pickle". Tip. I transfer my salad to a large jar with a fitted lid and pop it in the fridge. Everyone once in a while I turn the jar over to make sure the marinade is getting to all the mushrooms . Leave to marinate for at least an hour. The longer you leave the salad, the better it is. You can even make it the day before or in the morning if you are serving it in the afternoon.
Remove the mushrooms from the fridge. Before serving, stir through the cubed avocado, tomatoes (if using) and the green fennel fronds.
Serve with roasted sweet potato or with crusty bread.