Heat oil in a pot. Sauté onion and celery for 5 minutes.
Add garlic, chili, and cumin seeds. Cook 2 minutes.
Stir in ancho chili and tomato paste. Cook until paste darkens.
Add black beans, their liquid, and stock. Cover and simmer for 20 minutes, stirring occasionally. Adjust thickness with more stock if needed.
Spoon stew into bowls. Serve with tortillas and toppings: avocado, cheese, lime, onion, and cilantro.
