Preheat oven to 375°F. Lightly oil a baking sheet.
Toss diced sweet potatoes with olive oil, chili powder, cumin, salt, and pepper. Spread on baking sheet and roast for 20 minutes until tender.
In a skillet over medium heat, sauté onion and garlic until soft. Stir in black beans and half the enchilada sauce. Remove from heat.
Grease a 9x13 baking dish. Spread a thin layer of sauce on the bottom. Arrange tortillas, then layer roasted sweet potatoes, bean mixture, cheese, and sauce. Repeat layers.
Top with remaining cheese and cover with foil. Bake 25 minutes. Remove foil and bake another 10 minutes until bubbly.
Cool slightly before serving. Garnish with cilantro.