Instructions:
- Heat a large skillet over medium-high heat. Brown the ground beef, breaking it up. Drain off any excess fat. Return the meat to the pan.
- Stir in the chopped onion and cook until softened (about 3 minutes). Add taco seasoning and water/broth; simmer until slightly thickened. Set aside.
- In a large bowl, whisk together the milk and eggs until smooth. Set aside. This will be the binder.
- Lightly grease the bottom of the slow cooker insert. Spread half of the refried beans evenly across the bottom.
- Arrange half of the cut tortillas over the refried beans, slightly overlapping them to cover the base.
- Spoon half of the seasoned beef mixture evenly over the tortillas. Sprinkle with half (1/2 cup) of the Monterey Jack cheese.
- Top with the remaining tortillas, followed by the remaining refried beans, the rest of the seasoned beef, and the rinsed black beans.
- Slowly pour the egg/milk mixture evenly over the entire casserole. Sprinkle the remaining 1/2 cup of Monterey Jack cheese on top.
- Cover and cook on LOW for 6-7 hours or HIGH for 3-4 hours, until the liquid is mostly absorbed and the casserole is set.
- Fifteen minutes before serving, sprinkle the Sharp Cheddar cheese on top. Replace the lid and allow it to melt completely.
- Let the casserole rest for 10 minutes before scooping. Garnish with fresh cilantro and sliced jalapeños before serving.