Prepare mushroom caviar from champignons. Pour vegetable oil into a deep frying pan and add butter. Put on the fire. When the ingredients are combined into a homogeneous mass, put in the onion. Fry it, stirring, for 3 minutes. Add mushrooms and stir. Reduce the heat. Simmer the mushrooms with onions for 15 minutes until all the moisture evaporates. Add salt, pepper and stir. After 1 minute, remove the pan from the heat, let the caviar cool. Transfer the ingredients to a blender bowl and grind to the desired consistency. Add lemon juice. Stir the mixture.