Wash the peppers and remove the insides. Salt them on the inside.
Heat some butter and fry the finely chopped onion, ham and mushrooms, season with salt and black pepper.
In a separate container, fry 2 tablespoons of butter with the flour, dilute with a little milk. Add some of the white roux to the stuffing, along with the breadcrumbs, stir well and stuff the peppers.
Arrange them in a dish with the opening facing up if possible and sprinkle each opening with breadcrumbs and put a little piece of butter on top.
Put the peppers to bake until ready and serve with the remaining white sauce.