In a small skillet, heat about ½ inch of vegetable oil.
Test the oil for proper temperature by putting in a small piece of tortilla.
When the oil reaches the temperature where it immediately begins to bubble frantically over the tortilla as soon as it is put in the oil and the tortilla piece becomes crisp quite quickly, you are ready to cook the chalupa shells.
This is important because there is nothing worse than a soggy chalupa shell which results from the oil not being hot enough.
Fry tortillas completely flat on both sides until very crisp.
Drain and keep warm on a newspaper-covered cookie sheet in a warm oven.
Spread about ¼ inch thick layer of refried beans on each fried tortilla.
Top with a generous amount of grated sharp Cheddar cheese and about 1 tablespoon chopped onion.
Place assembled chalupas on a cookie sheet and brown under broiler until cheese melts.
Watch them carefully under the broiler, they can burn quite quickly.
Top with shredded lettuce and tomato and serve.