In a large bowl, toss together sweet potato cubes, olive oil, salt, and chili powder. Transfer to a baking sheet and spread out in an even layer. Roast for 10-15 minutes or until tender and slightly golden. Remove from the oven.
While the potatoes cook, add the black beans, water, garlic, and cumin to a small saucepan. Simmer over medium heat until the water is reduced by half. Remove from the heat and smash into a paste using a potato masher.
Meanwhile, lightly brush the tortillas with olive oil. Place them on a cookie sheet and bake at 450 degrees F for 10 minutes or until crispy, flipping once. Remove from the oven.
Top crispy tortillas with a heaping tablespoon of smashed black beans and some cheese. Return the tortillas to the oven for a few minutes, or until the cheese is melted.
Remove from oven and add roasted sweet potatoes, salsa, avocado, and green onion on top. Serve warm.