In a small skillet/frying pan, melt the beef suet/tallow over a medium heat then add the onion. Cook, stirring regularly, until it has softened but is not browning (if it starts to brown then reduce the heat).
Reduce the heat to low, add the oatmeal and stir to mix. Season with a pinch of salt and pepper. Allow to cook, stirring now and then, for around 15 minutes until the oats are golden and cooked through. They will be a bit nutty and have some chew, but should taste cooked. If you want them slightly softer, add a teaspoon or two of water or stock, and cover to steam, but it shouldn't really be needed.
Video
Notes
If you can't source suet or tallow (or prefer a vegetarian alternative), butter or shortening are the best options. You may find you need a little extra salt and pepper to give a bit more flavor, as a result.If you like, you can add a small amount of dried thyme or sage as the oats cook for a little additional flavor, though go light on these if you do - you still want the delicate oat flavor to come through.