Peel the onions, cut in half and cut into strips. Peel the garlic and cut into fine slices.
Heat the oil in a saucepan. Stir in the onions with garlic and thyme, fry over medium heat for about 5 minutes. Stir again and again.
In the meantime, cut the tomatoes into thin strips. Wash the lemon with hot water and finely grate the peel. Wash the salmon fillet and pat dry and season with the lemon zest, salt and pepper.
Heat the remaining oil with the butter in a pan. Fry the fish in it over medium heat for about 2 minutes on each side. Add the stock with tomatoes and balsamic vinegar to the onions. Season to taste with honey, salt and cayenne pepper. Arrange the salmon fillet with the onions on plates.
It goes well with white bread and rosé wine as a drink.