Remove the stems from the guajillo chiles and chile de arbol.
Cut the chiles lengthwise.
Remove and discard any seeds and veins from all the chiles.
Place them in a stock pot along with 4 cups water.
Bring to a boil.
Turn heat off and let the chiles sit in the hot water for 5 minutes, or until pliable.
Discard the water and place the rehydrated chiles in a blender.
To the blender, also add chipotle, tomatoes, ¼ onion, garlic clove, and 1 tablespoon salt.
Blend until smooth.
Taste for salt. Add more if necessary.
Strain the sauce.
Set aside until ready to use.
Dice the remaining onion.
Pat dry the fish.
Season the fish with the remaining salt and pepper.
Heat oil in a large skillet.
Saute the onion until translucent. About 1 minute.
Add the Diabla sauce. Cook for 1 minute.
Add the fish.
Cook for 4-5 minutes, depending on thickness of the fish.