Remove entrails and films from the fish carcass and rinse. Cut it into portions. Marinate it for at least 40 minutes. Peel and rinse the vegetables. Preheat the oven.
For the marinade, mix soy sauce with Dijon mustard, adding olive oil and pepper. Mix the portioned pieces of fish well with the marinade.
Cut the peeled carrots into thin slices, the onion into thin half rings, and the lemon into slices.
Cover the baking dish completely with foil. Place chopped carrots on the bottom and onions on top. Place marinated mackerel on the onion, place lemon between the pieces of fish, and pour lemon juice on top. Place the remaining vegetables on the sides, alternating.
Bake the fish in a preheated oven at 180 degrees for 25 minutes. Serve mackerel in portioned plates with garnish.
Prepare mashed potatoes or crumbly boiled rice as a side dish for the fish.