In a large pot, boil the flank steak in about 12 cups of water with 1 tsp of salt and 2 bay leaves for 2 hours. Ensure that the meat remains entirely submerged in the water. If a lot of the water evaporates within the 2 hours, add the remaining 6 cups of water. Remove the scum that rises to the top of the water as it boils.
Remove the steak from the pot and allow it to rest. Also, reserve the remaining cooking water.
Add the tomatillos, tomatoes, garlic, onion, sesame seeds, pumpkin seeds, dried chilies, and 1 cup of the reserved cooking water to a blender and blend until completely smooth, working in batches if needed.
Add the blended mixture, along with 2-2.5 cups of the additional reserved cooking water, to the large pot over medium heat and allow it to come to a simmer.
Shred the meat into medium-sized shreds (if you shred the meat too fine, it will become tangled in the stew) and add it to the pot along with the cumin, salt, pepper, and remaining bay leaf. Allow this to simmer for about 10 minutes (you’ll notice that the broth will begin to deepen in color).
Add the chopped potatoes and carrots, and simmer for an additional 12-15 minutes on medium heat, or until the potatoes and carrots are fork-tender.
Check for seasoning before serving with rice, beans, and fresh tortillas.
