Cook the spinach very quickly in simmering, salted water until wilted. Drain well and squeeze dry.
Thinly slice the goats' cheese.
Place a flour tortilla in a dry, non-stick frying pan and scatter with one quarter of the goats' cheese. Then add one quarter of the wilted spinach, and season to taste.
Place another tortilla on top to form a sandwich. Cook over a medium heat until lightly browned.
Turn once and cook the other side until the tortilla is lightly browned and the cheese has melted. Repeat with the remaining tortillas, goats' cheese and spinach.
Meanwhile, peel, stone and roughly chop the avocado. Toss with the lime juice, coriander, sea salt and pepper.
Serve the quesadillas on a plate cut in half or quarters with the avocado.