Preheat the oven to 400 degrees F, slipping in a cookie sheet at the same time.
For the sauce: Heat the oil in a saucepan on the stove and fry the onion, bell pepper and chiles. Add the salt and cook gently for 15 minutes, and once soft, add the chopped cilantro stalks.
Add the canned tomatoes then rinse the cans out with water and add this too. Spoon in the ketchup and let things come to a simmer, leaving the sauce (or salsa in keeping with the Mexican mojo) to cook while you get on with preparing the filling- about 10 minutes.
For the filling: Mix the drained beans and canned corn in a bowl. Add most of the grated cheese, reserving some to sprinkle on the top at the end, and mix together.
Start to assemble the lasagne by spooning about a third of the salsa into the bottom of your ovenproof dish and smear it about, then layer on 2 tortillas so that they cover the sauce overlapping slightly, like a Venn diagram.
Add a third of the beans and cheese mixture covering the tortillas, and then about a quarter of the remaining salsa and another 2 tortillas.
Repeat with another third of beans and cheese, and more salsa before layering on another 2 tortillas.
Finally, add the last layer of beans and cheese, nearly all of the remaining salsa and cover with the last 2 tortillas. Spread the very last bit of salsa over the tortillas and sprinkle with the remaining cheese.
Bake for 30 minutes, and let it rest for a good 10 to 15 minutes before slicing like a pizza, and eat with the avocado salsa.
Make Ahead Note: The lasagne can be assembled 1 day ahead. Cool the tomato salsa before assembling. Cover dish tightly with plastic and refrigerate until needed. Bake as directed in recipe, allowing an extra 5 to 10 minutes baking time and checking the lasagne is piping hot in centre before removing from oven.
Freeze Note: The assembled but uncooked lasagne can be frozen for up to 3 months. Wrap dish tightly in double layer of plastic and a layer of aluminium foil. Thaw overnight in fridge and cook as above.
Further Note: Leftovers should be covered and refrigerated as soon as possible. To reheat, put individual slices in ovenproof dish, cover with foil and reheat in oven preheated to 350 degrees F for 15 to 20 minutes. Check the lasagne is piping hot before serving. Wedges of leftover lasagne can be frozen for up to 2 months, tightly wrapped in plastic wrap and then either in foil or in resealable bag. Thaw overnight in refrigerator and reheat as above.
Halve the avocados, remove the pits, and then, using the tip of a sharp knife, cut lines down the flesh of each half, about a 1/4-inch apart and then the same across. Turn the avocado halves inside out, or otherwise release the pale green cubes into a bowl, then tumble in the scallion, chopped jalepenos, salt to taste, lime juice and most of the cilantro and toss gently to mix. Taste to see if any more salt is needed before scattering with the remaining cilantro and taking to the table.