Put the onion, jalapeno and garlic in a food processor and finely chop. Add the tomatillos, avocado and salt and pulse until chopped but still chunky. Transfer to a bowl and stir in the coriander.
Heat a hob or outdoor grill to medium-high. Mix the olive oil, chipotle or chilli powder and salt in a large bowl. Add the prawns and toss to coat. Grill the prawns until translucent, about one and a half to two minutes on each side.
Grill the tortillas until slightly charred and pliable, about 20 seconds per side. (Alternatively, wrap in a damp paper towel and heat in a microwave.)
Spoon the sauce on the tortilla, then top with about 2 or 3 prawns and a sprig of coriander. Serve 2 tacos per person, with a lime wedge on the side.