Combine oil, garlic, chilli, coriander in a small bowl, season to taste. Place mushrooms on a plate. Spoon over the marinade, turning to coat both sides. Stand for 10 minutes to allow mushrooms to absorb the marinade.
Put pancetta into a large non-stick frying pan over medium-high heat, cook 3-4 minutes each side until golden. Remove to a plate. Increase heat to high. Add the mushrooms stalk-side down, cook for 3-4 minutes on each side or until just tender.
Meanwhile, combine avocado, yoghurt, lemon juice in a bowl and season to taste.
Place mushrooms onto serving plates. Top with avocado and pancetta. Serve with toasted sourdough.