Using a large pot or saucepan, sautée the onion, green bell pepper and garlic in olive oil for 3 minutes. Add chopped tomatoes and sautée for another 2 minutes.
Add the cod chunks and let it absorb the flavors. Toss in the shrimp chunks and cook it for 2 minutes.
Add the mashed potato and combine. This is the stuffing for the piquillo peppers and should not be too liquid. Remove from heat.
Prepare the sauce by using a mixer or a food processor. Add all ingredients to a bowl and mix it until well blended and smooth.
Stuff the peppers one by one, place them on a skillet. Add the creamy sauce and turn on the heat.
They are ready to serve when the sauce starts to boil.
Arrange them on the plate, sprinkle chopped parsley and serve immediately.