Peel and finely dice the shallots. Wash basil, dab dry and cut leaves into fine strips. Wash lemon and grate dry. Grate half of the peel. Squeeze lemon.
Remove the skin (if present) from the salmon. Dice salmon and trout finely. Mix with shallot, basil, lemon peel and 4 tbsp. juice. Season to taste with pepper and salt if necessary
Heat the butter in portions in a pan, fry 2 slices of bread in it from each side until golden brown. Take them out and let them drip off on kitchen paper. Fry the remaining slices of bread in the remaining butter in the same way. Spread the tartar on the toasted bread and serve immediately