Preheat oven to 350 degrees.
In a heavy saucepan, melt butter. Stir in the flour, then add the milk. Cook while whisking until sauce becomes thick. Season with salt and pepper.
Separate the eggs. Mix the yolks and cheeses in the thick cream sauce. Fold in the fiddleheads.
Whip the egg whites and cream of tartar until firm. Gently fold in the cheese-fiddlehead mixture.
Butter 2 7-inch soufflé molds. Coat with flour. Shake any excess out. Pour souffle mixture into molds. They should be 3/4 full.
Bake 30 to 40 minutes.
You may serve this souffle with cheddar cheese sauce.