Peel and finely dice the challots. Wash basil, shake dry, cut leaves into strips. Wash lemon hot, dry and grate half the peel finely. Squeeze the lemon. Remove the skin (if available) from the salmon.
Dice the salmon and trout finely. Mix with shallots, basil, lemon peel and 4 tablespoons of juice. Season to taste with pepper and salt if necessary.
Melt the butter in portions in a pan. Fry 2 slices of bread on each side until golden brown. Take them out. Spread the tartar on the toasted bread and serve immediately.