To make the mole, heat the oil in a heavy pot and sauté the onion until translucent.
Add the garlic cloves and continue sautéing.
Prepare the serranos by cutting them into four pieces lengthwise. If a milder taste is desired, scrape out the seeds.
Add the serranos and pumpkin seeds and sauté until the pumpkin seeds are toasty and slightly browned.
Transfer the mixture into a large blender or food processor along with 2 cups of the fish stock and puree until thoroughly smooth.
Return to the pot and bring to a gentle boil.
Turn down the heat, cover the pot and allow to gently simmer.
Chop the romaine leaves into large chunks (if using spinach, no chopping is necessary) and place in the blender or food processor along with the tomatillos, cilantro and epazote, if using.
Blend while gradually adding the remaining 2 cups of stock until a thick, smooth consistency is achieved.
Pour this mixture into the pot, add 2 teaspoons of salt and simmer until thickened—roughly 20 to 30 minutes.
Taste and add more salt if needed.
To make the fish, heat the butter in a heavy frying pan until almost brown but not burned.
Mix the flour and salt and dust the fillets lightly.
Fry each side 2 to 3 minutes—taking care not to overcook.
Spread about ½ cup of mole onto each plate and top with a fillet. Serve immediately.