Slice the plaice into 2cm chunks and toss with the chipotle paste, lime juice and ½ tsp vegetable oil. Leave to marinate for 10 minutes.
In the meantime, heat the rest of the oil in a heavy skillet pan over a medium heat. Add the onion, pepper and a pinch of salt, stirring to coat the vegetables in the oil. When the onions start to brown, after 5 minutes, turn down the heat, stirring occasionally.
Add the plaice, and cook for 7 minutes until the fish flakes apart, and the vegetables are tender but still have some crunch.
Heat a grill pan and warm the tortillas.
Assemble the tacos with the vegetables, plaice and avocado slices. Top with coriander, and serve.