Cut the tuna steak into a 3cm square block, season and rub all over with olive oil
Heat non-stick fry pan and sear tuna over high heat on each side until golden but still pink in the middle. Set aside
Combine the mayonnaise, crème friache and cornichons in a bowl. Place 1 teaspoon of the mayo mixture into each tart case and place a piece of thinly sliced tuna on top. Garnish with a sprig of chervil and serve immediately