Heat the oil in a large pan, preferably non-stick; add the ground mince/mince beef and stir over a high heat, breaking it up as it cooks to ensure that it doesn't stay in large lumps; cook for about 5 minutes or until the beef has browned.
Add the taco seasoning and the water; bring to the boil; reduce the heat to low and simmer, uncovered, for about 5 minutes or until the sauce has thickened; stir in the corn and the green onions.
Guacomole: Meanwhile, cut the avocado in half lengthways; remove the seed and scoop the flesh into a bowl; add the lemon juice and mash the avocado roughly with a fork; cut the tomato in half and squeeze gently to remove the seeds; chop the tomato, then add it to the avocado mixture; season to taste with salt.
Place half the tortillas on a chopping board; spread the beef mixture evenly over the tortillas; sprinkle with cheese; top with remaining tortillas; using a pastry brush, brush the top of the tortillas with the extra oil (or use an oil spray).
Cook the quesadillas in batches in a heated sandwich press until the cheese has melted; or, add the quesadillas to a heated non-stick pan and cook until lightly browned on both sides, turning carefully.
Remove the quesadillas from the sandwich press or pan with a plastic spatula; place on a chopping board and cut into quarters with a sharp knife.
To keep the quesadillas warm while cooking the remainder, place them on an oven tray and place in a 120°C/100°C fan-forced oven/250°F/1/2 gas mark.
Serve the quesadillas with the Guacamole.