Clean the mushrooms and remove the gills inside.
Cook the mushrooms until tender on a grill pan (about 10 minutes), season with a little salt and pepper.
Place one mushroom cap side up.
Rinse and dry the lettuce leaves.
Peel the half avocado and slice.
Place lettuce, chicken, onion, tomato, and avocado on top and finish with parmesan.
Finally, pour over a pesto and place the other mushroom on top.
Combine herbs, oil, mustard, and vinegar in a mini chopper and blend into a pesto, optionally diluting with a little water to the right consistency.