Preheat oven to 350 degrees.

Melt 2 tablespoons butter in a shallow baking dish; place chicken breasts in the pan.

Bake, uncovered, about 30 minutes.

Meanwhile, over low heat, saute the onion and celery until tender in the remaining butter in a saucepan.

Dissolve cornstarch in chicken broth and add to sauteed mix, stirring until thickened.

Add yogurt, wine, lemon juice and pepper, stirring until blended.

Remove pan from oven.

Turn each chicken breast and cover with sliced mushrooms.

Pour sauce over all.

Bake, uncovered, for another 30 minutes or until chicken is done.