Mix in the flours, baking powder and salt in a large bowl. Make a well and add the oil.
Mix the oil into the flour with finger tips.
Using as much hot water as required, make a smooth pliable dough.
Keep this covered with a damp kitchen towel and allow dough to rest for 30 minutes - 1 hour.
Make 4 equal sized balls and roll out each one on a clean countertop, quite thin into a large circle or rectangle.
Using a cookie cutter or a sharp edge of a round cup or bowl, cut out around 1.5-2u201d circles. Make as many circles as you can from the entire dough.
Preheat the oven at 190xb0C. Line a baking tray with baking paper or a silicon mat. Place the tostadas in a single layer.
Brush vegetable oil on top of the tostadas and bake for 5-7 minutes or until the tops turn golden brown. They can burn very quickly, so keep a watch on the oven door.
Bake in 2-3 batches depending on the size of the baking tray and oven. Allow to come to room temperature and store in an airtight container until ready to use.
To assemble tostadas, top each one with some refried beans, charred corn salsa, pico de gallo and some grated cheese.
Serve immediately. You can also lay out the tostadas and a variety of toppings on the table so your family / guests can assemble it themselves.