Deseed the tomato (Try to get as much of the juice out) and cut the tomato "meat" to small cubes, about 3-5mm.
Chop the Scallion (Both green and white parts) to thin rings.
To cut the Avocado: Peel the Avocado, and cut it lengthwise, place the two halves face down and run your knife both horizontally and vertically at 1 cm apart lines to create 1 cm size pieces, then moving it to the salad bowl try to separate the pieces without meshing them.
Squeeze in juice from 1/2 a lemon (if it is a large lemon you might need less).
Chop the Chili very finely and add to taste (I like it very hot).
Season with Salt and pepper, mix well (gently, try not to mesh the avocado) and fix the seasoning (Salt, Pepper, Chili, Lemon)
Place one Tortilla on a plate. Grate (roughly) half of the cheddar cheese in an even layer. Top it with the finely sliced ham. Grate the rest of the Cheddar and close with a second Tortilla.
Grill the Quesadilla. This can be done on a hot nonstick large pan, on your grill or under the broiler in your oven. Make sure the Tortillas get a bit brown and become crispy (on both sides).
As soon as your Quesadilla is done, place it on a large plate, cut it (like a pizza) to 6 or 8 slices, top each slice with a large spoon of the avocado salad and serve.