2) Melt 1 tbsp of butter in a small frying pan or skillet. Carefully place a quesadilla in the pan and fry, turning once, until golden and little bubbles appear on both sides, 4 to 5 minutes in all. Repeat with the remaining quesadillas, wiping out the pan between batches if the butter burns.
3) While the quesadillas cook make the salsa. Halve the tomatoes and grate them on the largest holes of a box grater into a bowl, discarding the skins. Grate the onion into the tomato and stir in the coriander, hot sauce and salt, to taste.
4) Cut quesadillas into four wedges with a pizza wheel or knife and serve with the salsa and slices of avocado.