Wash mushrooms and cut into small cubes. Preheat a large frying pan with 2 tbsp of olive oil. Cook mushrooms until golden brown. Drain any water and set it aside.
Chop onion into medium/small cubes and cook in the pan over medium heat with olive oil until translucent. Set aside.
Clean herring under running water. Cut into medium cubes and set aside. Cut drained olives into quarters.
In a large serving bowl begin layering all of the ingredients starting with mushrooms and following with onions, pieces of herring, olives, a tablespoon of oregano and basil – sprinkled all over each layer, pepper and a quick drizzle of olive oil (just a bit). Continue layering until all herring is finished.
Store in the fridge tightly covered with foil and enjoy over the next 3 days!