Sprinkle sole lightly with salt and pepper. Fold fillets crosswise and place in a greased 8-inch-square baking pan. Top with onion and lemon slices; pour wine over all. Cover and bake in a 400 F. oven for 15 to 20 minutes or until fish flakes easily when prodded in thickest portion with a fork.

Remove from oven; discard lemon slices. Drain pan liquid into a measuring cup; add water, if necessary, to make 1/2 cup. Keep sole warm.

Melt butter in a small pan over medium-high heat; add mushrooms and cook until lightly browned. Stir in flour until bubbly. Gradually add cooking liquid from fish and cook, stirring constantly, until thickened. In a bowl, combine egg yolk with cream and beat; stir in a little of the hot liquid from mushrooms. Add egg mixture to mushroom sauce in pan and cook, stirring constantly, until thickened.

Lift sole onto a warm serving dish. Blend a little of the liquid that collected in fish pan into mushroom sauce to thin it slightly, if needed. Spoon sauce over fish. Serve immediately, garnished with parsley sprigs and lemon wedges.

Yield: 2 servings