Preheat barbecue plate on medium-high. Lightly spray both sides of the mushrooms with olive oil. Barbecue the mushrooms, stalk-side down, for 2-3 minutes then turn mushrooms over and barbecue further 2-3 minutes or until just tender, but still holding their shape.
Spread hummos over the toasted walnut bread. Top with mushrooms then crumble over the feta or ricotta.
Drizzle the rocket with olive oil and squeeze over the lemon. Season with salt and pepper and toss gently to coat then pile on top of the mushrooms. Serve.