In a large skillet over medium heat, add the ground beef and cook until browned. Drain any excess fat.
Add the diced onion and minced garlic to the skillet, cooking until the onion is translucent (about 3-4 minutes).
Stir in the black beans, diced tomatoes (with their juice), cooked rice, and taco seasoning. Mix well and simmer for 5 minutes. Season with salt and pepper to taste.
In a greased 9x13 inch baking dish, lay out 4 of the flour tortillas, overlapping them to cover the bottom of the dish.
Spread half of the beef mixture over the tortillas, then sprinkle with 1 cup of shredded cheddar cheese.
Layer the remaining 4 tortillas over the cheese, followed by the remaining beef mixture. Top with the remaining cheese.
Cover the dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Allow it to cool for a few minutes, then drizzle the sour cream over the top and garnish with fresh cilantro if desired.