1) Melt butter in a non-stick 25-cm saute pan over medium-high heat. Add the mushrooms and saute for 5 minutes. Meanwhile, beat together the eggs, double cream, salt, and pepper. Reduce the heat to lowest setting.
2) Add the egg mixture to the pan. Cook the eggs for 5 minutes, or until they start to set. Remove the pan from the heat. Sprinkle the cheese over the top of the eggs and place the pan under the grill. Grill for 5 minutes until the eggs have cooked and the cheese is melted. Remove the frittata from the pan onto a cutting board and sprinkle with chives.
Cut the into 4-cm pieces. Top each piece with a dollop of creme fraiche and caviar.