Place chiles in a heatproof bowl. Cover with boiling water and let sit until soft, about 30 minutes.
Combine chicken, quartered onion, whole garlic cloves, bay leaves, salt, and water in a large stock pot. Bring to a boil, then reduce heat to a low simmer. Cook until chicken is very tender, about 1 hour.
Remove chicken from broth and set aside to cool slightly. Strain broth through a fine mesh sieve into a heatproof bowl, wash out pot and dry thoroughly. Once chicken is cool enough to handle, shred into bite-sized pieces and set aside.
Heat 3 tablespoon of oil in the pot over medium heat. Add chopped onion and garlic and season with salt and pepper until just starting to brown, about 10 minutes.
Transfer vegetables to a blender and add masa, soaked chiles, oregano, and 1 1/2 cups of the reserved broth. Blend on high until smooth.
Return pot to medium heat and add the last tablespoon of oil. When shimmering add chile mixture (careful, it will spatter) and cook, stirring constantly, until boiling and thick, about 2-3 minutes.
Add reserved broth, chicken, potato, and bring to a boil. Reduce heat to a simmer and cook until potato is tender, about 20 minutes, stirring occasionally.
While soup is cooking mix together the sour cream, lime zest, juice, and salt in a small bowl.
Taste soup for salt and season as needed then serve in deep bowls with dollops of lime sour cream, jalapeño slices, cilantro, and a smattering of toasted pumpkin seeds.