Can't decide between salmon or tuna tartare? Parfait them.
Whip the crème fraÎche until thick, and stiff peaks are formed. Reserve in the refrigerator.
Combine the shallots and 4 tablespoons olive oil in a small saucepan and sauté until the shallots are translucent. Place the shallots in a bowl and add the capers, lemon zest, soy sauce, horseradish, coriander, and chervil. Mix until combined.
Divide the shallot mixture into 2 equal parts. Combine one half with diced tuna; combine the other half with ground salmon. Add 1 teaspoon salt and 1 teaspoon pepper each to the tuna mixture and salmon mixture and mix until all ingredients are combined.
Lightly brush the inside of 10 rings or molds with olive oili. Place waxed paper on a cookie sheet. Place molds on the cookie sheet.
Place 2 tablespoon of tuna mixture in each mold. Smooth the tuna mixture with back of spoon.
Add 2 tablespoons of the crème fraîche. Smooth with the blade of a flat knife, making sure that crème fraîche is level with the top of the mold. Last add 2 tablespoons of the salmon mixture and smooth. You should have 3 equal layers.
Chill in refrigerator for 2 to 3 hours. Place each ring or mold on a plate and gently remove mold, leaving parfait on plate. Serve with toast.