Heat the friendly fat in the bottom of your soup pot over medium/high heat while you toss the beef, 2 tablespoons of the arrowroot, salt, and pepper in a bowl to coat. When the oil is hot, carefully set the coated beef cubes in, searing the meat on all sides. Set the beef aside leaving the juices/oil behind in the pot.
Over medium heat, add another tablespoon of oil along with the onions and mushrooms. Sprinkle with a big pinch of sea salt and cook over medium/medium-high heat for about 7 minutes, until the mushrooms and onions are softened and caramel in color.
Add the garlic and cook for a minute, then add the coconut milk. Bring the coconut milk to a simmer for a minute or 2, and then add the last 2 tablespoons of arrowroot. Stir until everything thickens.
Add the broth, seasonings, and seared beef to the pot and bring to a simmer. Keep the soup at a low simmer over low-medium heat for 10-15 minutes.
Turn the heat off, and add the chopped spinach in to wilt, then and sea salt and pepper the soup to your taste.