In a large pan, heat the olive oil on medium low heat. Add the onions and butternut squash and coat them in the oil. Cook until the onions and squash have softened, around 20 minutes.
Add the corn starch, chili powder, cumin, garlic powder, onion powder, salt and lime juice to the pan. Cover with one cup of water. Continue to cook until the squash is completely softened and the sauce has thickened, around 10 minutes.
While the squash mixture is cooking, add the black beans to a small saucepan to heat.
Heat corn tortillas on the burner of a gas stove on low heat. Fill the tacos with the black beans, butternut squash mixture, spicy serrano sauce and garnish with sliced serranos, cilantro and crumbled queso fresco (optional)