POSITION racks in top and bottom thirds of oven. Preheat to 400F. Stack tortillas and slice into 8 wedges each. Arrange on 2 baking sheets, then brush both sides of tortillas with 2 tsp oil. Bake in top and bottom thirds of oven, switching and rotating sheets halfway through, until golden, 5 to 6 min. Transfer to a rack. Preheat broiler.
HEAT a medium saucepan over medium- high. Add remaining 1 tsp oil, then garlic and cumin. Stir constantly until fragrant, 1 min. Add beans and water. Continue cooking until beans are soft and most of liquid is absorbed, 5 to 7 min. Mash with a potato masher until creamy and thick. Stir in salt.
ARRANGE baked tortillas on an ovenproof plate or dish. Layer with 1/4 cup cheddar, then beans and remaining 1/4 cup cheddar. Broil in top third of oven until cheese is melted, 1 min. Top with avocado, sour cream, tomato, green onion, jalapeno and cilantro.