Chill in the refrigerator just until the butter is cold. Butter the ramekins again to ensure easy unmolding.

Prepare béchamel sauce: Melt butter in saucepan. Add flour and stir over low heat for two minutes. Remove from heat and whisk in warm milk. Return to medium heat and bring to a boil, whisking until mixture thickens. Remove from heat.

Place béchamel in food processor. Add cheeses and process until smooth. Add eggs, salt and pepper and process again until smooth. Divide mixture into ramekins. Put the ramekins in baking dish and pour in enough simmering water to cover 2/3 up the sides of the ramekins.

Cover baking dish with foil and bake until just set, about 15 minutes (or until knife inserted in center comes out clean).

While flans are baking, lightly sauté mushrooms and set aside. Let rest for two minutes.

Run a knife around the sides of the ramekins and tap the sides to loosen the flans. Unmold onto appetizer plates. Serve garnished with the sautéed mushrooms.