In a large pot over medium-high, heat the lard until shimmering. Add the onion and cook, stirring, until softened, about 5 minutes. Add the garlic, jalapeños, cumin and cilantro stems, then cook, stirring, until fragrant, about 30 seconds.

Stir in the tomatoes, tortilla chips, broth and sugar. Bring to a boil, then cover, reduce to medium and cook, stirring, at a simmer, until the tomatoes have softened and their skins begin to peel away, about 10 minutes.

Remove the pot from the heat and cool, uncovered, for 5 minutes. Using a blender and working in batches so the jar is never more than half full, puree the mixture until smooth; transfer each batch to a large bowl.

Wipe out the pot, then pour in the puree. Cook over medium, stirring often, until heated, about 5 minutes. Taste and season with salt and pepper, then stir in half of the chopped cilantro leaves.

To serve, add tortilla chips to individual bowls, then ladle in soup. Sprinkle with the remaining chopped cilantro leaves and top with sour cream and avocado (if using).