Heat oil in a large nonstick skillet over medium-high heat. Add onion; cook for 3 minutes or until tender. Stir in beef and next 3 ingredients. Cook until beef is no longer pink.
Fold 2 (17x12") sheets of aluminum foil into 2 (17x2) strips. Arrange strips in an "X" pattern in a lightly greased 4 quart round slow cooker, allowing foil to extend 1 inch beyond edges of slow cooker.
Spread about 1/3 cup refried beans on 1 side of each of 5 tortillas. Place 1 tortilla, bean side up, atop foil "X" in slow cooker. Spoon 1 cup burger mixture over beans; top with 1/3 cup salsa and 1/2 cup cheese Repeat layers 4 times. Top with remaining tortilla, and sprinkle with remaining 1/4 cup cheese. Cover and cook on LOW 3 hours or until cheese melts and edges are bubbly.
Remove insert from slow cooker; let stand, uncovered, 15 minutes. Grasping ends of foil strips, carefully transfer pie to a serving plate. Carefully remove foil strips. Cut pie into wedges, and serve with desired toppings. A nice guacamole and tomato salad with some yellow rice goes well with this.