Wash and clean the fish. Put them at the bottom of an oven dish and leave them aside.
Then, start preparing the sauce. First, clean the onions and garlic and chop them finely. Put them in a deep pan and saute with a little water and oil.
Once they soften, add the mashed tomatoes. Add salt and a pinch of sugar to mask the acidity of the tomatoes.
Leave the sauce to boil until some of the water evaporates. Dice the pickles finely and add them to the sauce along with the peas.
If you'd like it to be thicker, add 1/2 tbsp of flour with a little water. When the tomato sauce is ready, sprinkle with ground black pepper and finely chopped fresh celery.
Distribute the sauce over the fish and place the oven dish to bake in the oven at 374°F (190 °C).