In a large pot or Dutch oven, heat the vegetable oil over medium heat.
Add the chicken pieces and cook until browned on all sides. Remove the chicken and set aside.
In the same pot, add the chopped onion and minced garlic. Sauté until the onion is soft and translucent, about 5 minutes.
Stir in the chopped tomatoes and bell pepper. Cook for another 5 minutes until the tomatoes start to break down.
In a dry skillet over medium heat, toast the pumpkin seeds, sesame seeds, guajillo chilies, and pasilla chilies until fragrant, about 2-3 minutes. Be careful not to burn them.
Transfer the toasted seeds and chilies to a blender or food processor. Blend until finely ground. Add a little water if needed to make a smooth paste.
Stir the ground spice paste into the pot with the onions, garlic, tomatoes, and bell pepper. Cook for 2-3 minutes to combine and let the flavors meld.
Stir in the ground cumin, ground cinnamon, dried oregano, black pepper, and cayenne pepper (if using).
Pour in the chicken broth and bring to a boil.
Return the browned chicken pieces to the pot. Reduce the heat to low, cover, and simmer for about 45 minutes to 1 hour, or until the chicken is tender and cooked through.
Stir in the chopped cilantro.
Adjust seasoning with salt to taste.
Enjoy your meal!