In a large skillet, melt 3 tablespoons of the butter over high heat. Add the mushrooms, onion and celery and cook, stirring occasionally, until the mushrooms are browned, about 6 minutes. Lower the heat to medium, sprinkle the flour into the skillet and cook, stirring, for 1 minute. Stir in the chicken broth, bring to a simmer and cook, stirring, until thickened, about 2 minutes. Stir in 1/3 cup of the milk and simmer for 1 minute more. Remove the skillet from the heat, add the lemon juice, and add salt and pepper to taste. Pour the gravy over the chicken in the baking dish.