Bring a pot with 18 cups of water to a boil over medium-high heat and heavily season with salt. Add the bay leaves and meat, and allow this to simmer for about 15 minutes.
Skim off and discard any scum that rises to the top, then lower the heat to medium and place the lid on top of the pot, ensuring that there is a small gap for steam to escape.
Allow this to simmer for about 2 hours, or until the meat is fork tender. Then transfer the meat to a cutting board and allow it to cool as you work on the next few steps.
Add the tomatillos, tomatoes, garlic, onion and dried chilies to the pot and allow this to simmer for 15 minutes, or until everything is cooked through.
Transfer the tomatillos, tomatoes, garlic, onion, chilies and bay leaves to a blender and add the sesame seeds, pumpkin seeds and cumin, along with 1 cup of the cooking water, and blend until completely smooth, about 30 seconds.
Transfer the blended sauce, along with an additional 2-2 1/2 cups of the cooking water, to an empty pot over medium heat.
Shred the meat into thick shreds (you don’t want them to be too thin because the meat will continue to pull apart as it cooks) and add it to the pot as well. Allow this to simmer together until the sauce deepens in color, about 10 mins.
Add the potatoes and carrots, and season with salt and pepper to taste. Allow everything to simmer with the lid on for an additional 12-15 minutes on medium-low heat, then serve with rice and tortillas.
Enjoy!