In a large skillet over medium heat, add the ground beef. Cook until browned, breaking it apart with a spatula as it cooks. Drain any excess fat and return the beef to the skillet.
Sprinkle the enchilada seasoning over the cooked beef. Stir in ½ cup of the enchilada sauce, corn, black beans, green chilies, and chopped green onions. Mix well and let it simmer for about 5 minutes.
Preheat your oven to 375°F (190°C). Take 6 slices of bread and lay them out on a baking sheet. Spoon a generous amount of the beef mixture onto each slice. Top with a handful of Mexican cheese, and then place another slice of bread on top.
Drizzle the remaining enchilada sauce over the top of each sandwich and sprinkle with more cheese. Bake in the preheated oven for 15-20 minutes or until the bread is golden brown and the cheese is bubbly.
Remove the baking sheet from the oven and let the melts cool for a couple of minutes. Top each melt with diced avocado.
