Preheat the oven at 350°F. Peel and dice 5 onions, slowly cook in a frying pan with some olive oil.
When the onions are translucent and start to get golden, add a can of diced tomatoes, oregano and Espelette chili pepper flakes, salt, pepper to taste and tsp or so of sugar. Cook slowly for about 15 minutes to remove as much moisture as possible.
Line a pie plate with tinned sardine filets (Cut the fish in half by inserting a knife in the belly cavity and lifting the filet. Remove the spine. Remove the bones and the scales. You can leave the skin on if you dare) pack them well.
Cover the fish with the onion and tomato mix, put the puffin pastel on top, cut half an inch around the pie plate and tuck the border into the pie plate. Poke some small holes and press the crust to remove air pockets. Bake for 35 to 40 minutes.
Cool for 10 minutes in the pan. Put a plate on the dish and flip it. Beware of juices! Lift the pie plate and voilà! Serve hot with a side of salad.