Heat the milk and pour it over the breadcrumbs and oatmeal. Let it stand for 10 minutes, then beat in the eggs. Mix the suet, herbs and seasoning with the flour, add the onions, and mix into oatmeal mixture. Beat thoroughly, adding a little extra milk if necessary. Bake for about 1 hour, and serve with pork or goose in the same manner as a Yorkshire Pudding.