Simmer wine, shallots, bay leaf, parsley, salt and pepper, uncovered, 5 minutes in a large enamel or stainless steel saucepan.
Add scallops, mushrooms, and enough water to cover, and simmer, covered, 3-4 minutes until scallops turn milky white. Drain, reserving cooking liquid.

Set scallops and mushrooms aside to cool, return liquid, bay leaf and parsley to saucepan and boil rapidly uncovered until reduced to 1/4 cup.

For the sauce, melt butter in a small saucepan and blend in flour. Strain reduced cooking liquid and add along with cream.

Heat, stirring constantly, until thickened and smooth. Spoon a little hot sauce into yolks, then return to pan; set over lowest heat and cook and stir 1 minute; do not allow to boil.
Mix in lemon juice, salt and pepper. Pour over scallops and mushrooms and toss to mix.

Spoon into 3 to 4 well-buttered very large scallop shells or into individual au gratin dishes; mix topping and sprinkle evenly over each.

Preheat broiler. If you have not made them ahead and chilled them, broil 5" from heat 4-5 minutes until bubbly and dappled with brown.