Trim the fiddleheads and wash by rinsing in a sink of cold water up to 4 times. Drain. Cook in a pot of boiling salted water until tender, about 10 minutes. Drain and immediately plunge into a basin of ice-water to set the color and stop the cooking. Drain, and set aside. Heat the chicken stock with the saffron to boiling. Turn off the heat. In a bowl, beat together the yolks and lemon juice. Whisk a ladleful of the hot stock over the yolk mixture. Add another. Then whisk the egg mixture back into the stock. Stir in the cream. Taste and adjust the seasonings. Add the fiddleheads and gently reheat, without boiling. Serve.
If it boils, it will curdle.