Preheat the oven to 400°F. Toss the onion, poblano chile, zucchini, and sweet potato with the olive oil, salt, black pepper, cumin, oregano, paprika, and cayenne. Spread into an even layer on a large rimmed baking sheet. Roast for about 20 minutes, until the vegetables are tender and lightly browned. Remove from the oven and stir in the black beans and lime juice.
Whisk together the buttermilk and sour cream in a small bowl. To serve, divide the roasted vegetables between the tortillas. Top each taco with the crumbled cotija cheese and a drizzle of crema.