Heat the olive oil in a frying pan and once hot, add the onions. Cook for 2-3 minutes or until starting to soften, then add the mushrooms and a big pinch of salt and pepper and cook for a further 7-8 minutes until cooked through.
Remove from the heat and stir through the tarragon.
Toast the bread, then top with the mushrooms and avocado and serve with a handful of watercress.
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