Mix the flour, milk, and baking powder together. Set aside in the refrigerator for four hours or overnight.
When ready to make the crepes, heat half the butter in the pan. Pour a scant 1/4 cup in the heated pan and immediately rotate the pan until a thin film covers the bottom. Cook until light brown, turn, and cook the other side. Stack with waxed paper between eat. Keep covered. Makes twelve crepes.
Heat the other half of the butter in the pan. Sauté the mushrooms in the melted butter.
In another pan, sauté the onion. Stir in the tablespoon of flour and gradually add the milk, stirring until bubbly and thick. Fold in the mushrooms, ham, and parsley.