See also: mushrooms, cheese, onions & garlic. Fried pieces of whatever cured pork is to hand, especially if you're having an egg in it. Flakes of smoked fish also work well alone or with egg - fling in some chives & chillies for zing & colour.

Tbh though, a bit of salt was the traditional Scottish way of having oatmeal, with black pepper making an appearance once spices became widely available.