Poach the brains by simmering them in the stock and white wine for 20 minutes.
Remove the brains.
Melt the butter in a skillet over medium heat and saute the onions and mushrooms until the onions are lightly browned—about 10 minutes.
Stir in the flour, season well with salt and pepper, and moisten the vegetables with about half of the poaching liquid.
Stirring frequently, simmer for about 15 minutes.
Slice the brains and arrange them in a shallow baking dish.
Pour the onion mixture over them.
Bake in a preheated 400° F. [200° C] oven for about 15 minutes to finish cooking the brains and reduce the sauce.