Make the refried beans. Add about 1 tablespoon of oil to a large skillet. Sauté onions in the skillet over medium heat. When onions are translucent and tender, add canned chili beans to the skillet (with the bean liquid from the can). Heat the beans until warm, and then use a potato masher or spatula to mash the beans as they're cooking. Continue to cook the beans down and mash until desired consistency, about 5-8 minutes. Then remove the skillet from the heat and set aside. *Beans will continue to thicken as they cool. You may need to reheat slightly before assembling the Mexican pizzas. *TIP - If beans get too thick, you can thin them with a bit of water or stock.
Cook the ground beef. Brown the ground beef on a griddle or in a skillet. Add taco seasoning and follow package instructions to prepare taco meat. When meat is done cooking, drain off excess fat if needed, and transfer to a bowl until ready to assemble the pizzas.
Fry the tortillas. While the beef is cooking, add high-heat cooking oil to the griddle, allowing it to pool in the corner for shallow frying. If using a skillet, add just enough oil to cover the bottom of the skillet. When the oil is heated, carefully lay one flour tortilla in the hot oil. Shallow fry on the first side until golden brown. Then flip to the second side and continue to fry until golden brown. *Total frying time is about 60-90 seconds per side, depending on your oil temp. When tortillas are done frying, transfer to paper towels or a wire cooling rack and lightly salt.
Assemble the bottom tortilla. Spread about ⅓ cup of refried beans over one fried tortilla. Top with about ¼ of the taco meat.
Assemble the top tortilla. Spoon about 2-3 tablespoons of Mexican Pizza sauce on one fried tortilla. Top with shredded cheddar and Monterrey Jack cheese. Sprinkle on diced tomatoes, sliced black olives, and sliced green onions.
Heat Mexican pizzas on the griddle or in the oven. *See notes for oven instructions. Place the two prepared tortillas (top and bottom) on the griddle to warm. Cover the top tortilla with a melting dome for the cheese to melt.
Serve Mexican pizzas. When the cheese is melted, remove the top and bottom tortillas from the griddle. Stack the top tortilla on top of the one with the beans and beef. Allow to cool slightly, cut into quarters and serve.