Heat a heavy griddle pan on the stove. Slice the bottom off each mushroom, then place the tops on the griddle, flat side down. Wrap some string around the thyme to make a brush. Put the brush end with a large pinch of salt in the mortar and then bash, adding about 5 teaspoons of extra-virgin olive oil and the white wine vinegar. Add a bit more salt and pepper.
Using the thyme bundle as a brush turn the mushrooms over and brush with the thyme oil, keep dabbing them while they are cooking. When the mushrooms are cooked, place them on a serving plate and sprinkle them with the parsley, chilli, and lemon zest. Squeeze over the lemon juice and drizzle with extra-virgin olive oil. Add a little balsamic vinegar, if you like.