In a large pot, melt the butter. Add the onion and carrots into butter and sauté until softened. (Keep heat on medium, since the butter can burn.) Add green peas and seasonings and cook a few more minutes.

Add water and bring to a boil. Lower heat to medium and cook for about 45 minutes. (Note: be aware that the soup will expand a bit after you add the oatmeal, so don’t fill your pot up to the top!)

Add oatmeal, stir, and cook for 15 minutes more.