Mix thinly sliced red onions with lime juice, zest, honey, and salt in a small bowl. The lime juice gently softens the onions while adding a vibrant tang. Set aside to let flavors develop while you cook.
Heat olive oil in a large pot over medium heat and cook the garlic until fragrant. Stir in tomato paste, smoked paprika, cumin, and chipotle chili paste, cooking briefly to bring out their aromas and deepen flavor. This builds a rich flavor base for the chili.
Toss in the diced sweet potatoes, then add the diced tomatoes and broth. Cover and simmer until the potatoes are soft and tender to the fork. Add water or extra broth if it seems too thick.
While the chili cooks, combine Greek yogurt, cilantro, avocado (if using), salt, pepper, and lime juice in a blender. Process until smooth and adjust seasoning to taste — this sauce will add a refreshing richness.
Once sweet potatoes are tender, stir in the black beans with their liquid and simmer briefly until the chili thickens. This step ensures the beans stay plump and flavorful.
Ladle the hot chili into bowls, drizzle with herby yogurt, and sprinkle pickled onions on top. Serve with warm tortillas or rice, and add cheese or extra cilantro for presentation.