Place the ground beef in a large deep skillet and cook over medium-high heat until evenly browned; drain off excess fat. Then add the taco seasoning mix, minced garlic, chili powder, cayenne pepper and water, stir well, reduce the heat to low and let it simmer for about ten minutes so the flavours meld and the liquid reduces.
While the meat mixture simmers, preheat your oven to 375 °F (190 °C) and spray or grease the bottom of your 9×13‑inch baking dish with cooking spray.
Place six of the 6‑inch corn tortillas into the bottom of the prepared dish. Spread one-third of the salsa over the tortillas, then layer half of the cooked meat mixture evenly on top. Sprinkle half of the green onions over the meat, then drop dollops of half of the sour cream across the top. Follow with half of each shredded cheese (Cheddar + Monterey Jack).
Repeat the layers: place another six tortillas, cover with the remaining salsa, remaining meat, remaining green onions, remaining sour cream dollops, and cheese (half remaining each). Then top with the final six tortillas, spread remaining salsa over them, and sprinkle with remaining cheese for a golden top.
Bake in the preheated oven for about 30–45 minutes, or until the cheeses are melted throughout and the top layer is golden brown. Let it stand for a few minutes after removing from the oven to make it easier to slice and serve.