Melt the butter in a medium pot over low heat. Add the onions, scallions and cook, stirring occasionally for about 5 minutes. Add the garlic, cumin and cook for about 2 minutes more.
Add the broth and bring to a boil. Add the oatmeal, potatoes, milk, salt and pepper to taste. Reduce heat to low, cover and simmer for about 30 minutes or until oats are tender. Place in the blender for a smooth texture.