Mackerel in the oven with mustard and mayonnaise

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A very tasty and quick recipe for baking mackerel in mustard and mayonnaise in the oven. I added a little soy sauce instead of salt, it goes well with the rest of the ingredients.
You can pour lemon juice over the fish carcass or add it to the sauce - it’s equally delicious. Necessary ingredients for cooking mackerel in the oven with mustard and mayonnaise. Do not defrost the fish, this will make it easier to clean.
Ингредиенты
Large mackerel - 1 pc.
Mustard - 1 tsp.
Mayonnaise - 1 tbsp.
Soy sauce - 1 tbsp.
Lemon juice - 1 tbsp.
Pepper, h.m. - taste
Sweet onion - 40 g
Рецепт
1. In a deep bowl, mix mayonnaise, not too spicy mustard, lemon juice and add soy sauce to taste instead of salt. You won't need more than a tablespoon. Set the sauce aside for now.
2. Cut off the head and tail of the mackerel, remove the entrails and the black film inside the belly, and wash the fish. Now you can leave it until completely defrosted.
3. When the carcass is completely thawed, make transverse cuts along the entire back on one side. Cut the onion into half rings, insert it into each cut and belly. Generously brush the mackerel with the mustard and mayonnaise sauce prepared earlier.
4. Transfer the fish to a baking dish, preferably on foil if it is not ceramic, and place in an oven preheated to 200 degrees for 20-25 minutes in non-convection mode.
5. Oven-baked mackerel with mustard and mayonnaise is ready to serve. Ruddy, juicy and very tasty fish, I recommend it!
6. An excellent side dish would be boiled potatoes or mashed potatoes.