Preheat the oven to 350°F (175°C) and lightly spray a 13 x 9-inch casserole dish with cooking spray.
Heat 1 Tbsp oil in a large skillet over medium-high heat. Add the finely chopped 1/2 small yellow onion and sauté until softened, about 3–4 minutes.
Add 1 pound ground beef and break it into small pieces with a spatula. Cook until mostly browned, then drain excess fat if desired.
Stir in the 4 minced garlic cloves, 1 Tbsp chili powder, 1/2 cup broth or water, and 1/2 tsp sea salt. Bring to a boil, then simmer, stirring occasionally, until most of the liquid has evaporated and the mixture is thickened.
Spread 1/4 cup of the enchilada sauce evenly over the bottom of the prepared baking dish.
Layer four corn tortillas over the sauce, allowing slight overlap as needed.
Spread half of the seasoned ground beef over the tortillas, drizzle with 1/2 cup enchilada sauce, and sprinkle with one-third of the grated 8 ounces Colby Jack cheese.
Add another layer of tortillas, spread 1/2 cup enchilada sauce over them, top with the remaining meat, and sprinkle with a second third of the cheese.
Place the remaining tortillas on top, spread the remaining enchilada sauce evenly, and top with the remaining cheese.
Bake on the center rack for 20–25 minutes, until the cheese is melted and bubbly.
Let the casserole rest at least 10 minutes before slicing to allow it to set.
Top servings with diced tomatoes, diced avocado, sour cream or Greek yogurt, and fresh cilantro as desired.
