Brown ground beef with onion and season to taste. Pour in sauce and simmer over low heat. Add water if mixture gets too dry. Heat refried beans in a saucepan. Add cheese and stir in till melted. Keep warm. Heat tortillas in microwave for one minute. Spread a small amount of beans on each tortilla.
Making refried beans in the slow cooker is much easier than you might think. You simply dump beans in. Add onions, garlic, and some spices and cover with water. Then turn the slow cooker on high and walk away. Hours later the beans are tender and ready to smash.
Traditionally, for Mexican style refried beans, pinto beans are used. But black beans are also wonderful prepared this way too, as are cranberry beans. In a pinch, I've even used white navy beans to make refried beans for tostadas. I just add some chipotle powder to them for seasoning. But usually we use either pinto beans or black beans.