Saute onion and garlic in butter in a large pan or pot until very soft. Add mushrooms, and cook until they've softened and released some of their juice. Add carrot, lentils, walnuts, stock, and seasonings (everything but the beans and oatmeal). Simmer (covered) for 5 - 10 minutes. While that's cooking, roughly mash the kidney beans. Add the beans and the steel cut oats to the pan, stir well to combine, and continue to simmer for 20 more minutes (covered). Stir the mixture regularly, as it has a tendency to stick to the bottom. It will become very thick, but it is meant to become a stiff mixture, as the oats will soak up the moisture, bind everything together, and give the loaf its structure. When the 20 minutes is up, stir again, remove from the heat, let sit for 10 minutes (the oats may still be somewhat al dente).

Grease a 9 x 5 loaf pan (a strip of parchment overhanging the sides will make for easy removal), and pack oat mixture into pan when ready, smoothing the top. Brush with more butter or oil, if desired.

Bake at 375 degrees for 30 minutes, until firm and lightly browned.

You can also make this in muffin pans, for easy portioning (and everyone gets a piece with a nicely browned top).