Heat the oven to 400°F. Lightly coat a 2-quart baking dish with a little of the olive oil so the casserole does not stick.

Set a large skillet on the stove over medium heat. Add the remaining olive oil and let it warm for about 30 seconds.

Add the chopped onion to the skillet with a small pinch of the salt. Cook for 4 to 5 minutes, stirring now and then, until the onion looks soft and lightly golden at the edges.

Add the sweet potatoes to the skillet. Stir well so the pieces get coated in the oil and onion.

Cook the sweet potatoes for 4 minutes, stirring once or twice, until they start to lose their raw look on the outside.

Add the garlic to the skillet. Cook for 30 seconds, stirring, just until it smells fragrant.

Sprinkle the cumin, the remaining salt, and the black pepper into the skillet. Stir for 15 seconds so the spices coat the vegetables.

Pour 1 cup of the enchilada sauce and 1/2 cup water into the skillet. Stir to combine.

Cover the skillet and cook for 8 to 10 minutes, stirring once halfway through, until the sweet potatoes are almost tender when pierced with a fork.

Add the drained black beans to the skillet. Stir and cook uncovered for 2 minutes, until the beans are heated through and the mixture looks thick and saucy rather than watery.

Spread 1/2 cup of the remaining enchilada sauce across the bottom of the prepared baking dish.

Arrange about one third of the torn tortillas over the sauce in the baking dish. It is fine if the pieces overlap.