Pipianes are one of the master sauces in Mexican cuisine. Unlike moles, they are seed- and nut- forward but are still flavored with toasted dried chiles. In my version, I use freshly roasted pumpkin purée to create a seasonal and slightly sweet and nutty sauce that is perfect for your favorite pan-fried fish, tofu or roasted vegetables.
Heat a large skillet over medium-high heat and toast the pepitas and pecans, tossing frequently, until very fragrant and browned in spots, 3 to 4 minutes; transfer to a large heavy pot.
Toast the ancho chiles in the same skillet, pressing them down with a metal spatula to make contact with the skillet, until lightly toasted on both sides, about 30 seconds total. They will burn easily so press down and turn them quickly. Transfer to the pot with the pepitas and pecans.
Add 4 tablespoons of the oil to the pot with the pepitas, pecans and anchos, and heat over medium-high heat. Add the onion, oregano, cinnamon, cumin and half of the garlic and cook, stirring occasionally, until the vegetables are tender and beginning to brown, 5 to 7 minutes. Add the stock, pumpkin and salt and bring to a boil. Reduce to a simmer, cover and continue to cook until the seeds and chiles have softened and the flavors have come together, about 30 minutes. Remove from the heat and let sit covered for 20 minutes to cool slightly before blending.
Working in batches if necessary, carefully blend the mixture until completely smooth. Transfer to a large bowl; stir to combine each batch. Keep warm while you cook the fish.
Meanwhile, season both sides of the fish with salt and heat the remaining 2 tablespoons oil in the same large skillet over medium-high heat. Cook the fish in a single layer, undisturbed, until golden brown, about 2 minutes; flip and cook the other side, undisturbed, until golden brown and cooked through, about 2 minutes. Remove to a plate.
Add the butter, chiles de arbol and remaining garlic to the same skillet and cook, scraping up any browned bits from the bottom of the skillet, until the garlic and butter just begin to brown, about 60 seconds. Transfer to a small heatproof bowl.
For each serving, spread about 3/4 cup of the pipian on a plate, top with a fish fillet, a chile de arbol, a few slices of toasted garlic and a drizzle of the browned butter. Refrigerate the leftover pipian in an airtight container for up to 3 days or freeze for 3 months. Reheat gently.