Bring a medium saucepan of water to a boil. Add the cubed sweet potato and cook until fork-tender, about 12-15 minutes. Drain well and mash until smooth; set aside. Preheat the broiler. Set aside 1 tbsp of chopped onion for garnish.
In a separate medium saucepan, heat 1 tbsp of olive oil over medium heat. Add garlic and the remaining chopped onion; cook, stirring often, until softened, 3 to 5 minutes. Stir in cumin, chili powder, and salt, cooking for 30 seconds until fragrant. Add cornstarch, stirring constantly, until the mixture becomes bubbly and thickened, about 4 minutes. Stir in the mashed sweet potato, whisking until smooth. Fold in the diced tomatoes and green chiles along with half of the shredded cheese.
Spoon the mixture into a shallow 3- to 4-cup baking dish and sprinkle the remaining cheese evenly over the top. Broil in the preheated oven until the cheese melts, about 2 minutes.
Garnish with reserved onion, and if desired, sliced jalapeño and cilantro. Serve warm with tortilla chips, crackers, or sliced veggies. Enjoy!