Heat oven to
400F (205C). Butter a shallow baking dish, about 10 x 6 x 2 inches (25 x
15 x 5 cm). Cut onions in half, then put cut sides down on board and slice
paper-thin. Put cut onions in a bowl, cover with boiling water and let
stand 5 minutes. Drain well. Heat 2 tbsp butter and oil in large heavy
skillet. Add onions and cook gently about 15 minutes, stirring. Onions
should be limp but not brown. HEat 1/4 cup butter in saucepan. Sprinkle in
flour and stir to blend. Remove from heat and add milk all at once. Add
salt, pepper, mustard and cayenne seed and stir to blend. Return to
moderate heat and cook until boiling, thickened and smooth, stirring
constantly. Stir in cheese and cream.
Cut eggs in
halves and lift out yolks. Put yolks into a coarse sieve and slice whites
thinly. Put 1/3 of the onions in the prepared baking dish. Sprinkle with
1/3 of the egg whites then press about 1/3 of the egg yolks through the
sieve over the egg whites. Sprinkle with 1 tbsp (15ml) of the parsley.
Repeat these layers twice more. Pour cheese sauce over all. Bake about 15
minutes or until bubbling well and lightly browned.