While you are waiting for the oven to warm up, cut the avocado in half, remove the stone, scoop out the flesh and place in a small bowl. Lightly mash with a fork, then add the lemon juice and extra virgin olive oil, season then set aside.
Place the sourdough slices on a large baking tray, drizzle over the olive oil, then put in the oven for 8 to 10 minutes, or until crispy and toasted, remove from the oven and leave to cool for 5 minutes.
Once cooled break each piece of toast into 4 pieces, then spread with the avocado. Blanch the watercross for at least 15 seconds in boiling water and allow to cool. Top each piece of toast with radish, red onion and watercress. Serve at once.