We haven't made this in a slow cooker, but I'm guessing that step 6 could easily be done in a slow cooker. Tostadas are crispy fried corn tortillas. They are sold packaged and can often be found in the same section of your grocery store as fresh tortillas, or can be found at Mexican markets. You can make your own by frying stale corn tortillas (or tortillas that have dried out a bit in a warm oven), in hot vegetable oil until stiff.