Pat the meat dry and sprinkle generously with salt and pepper.
Melt a generous amount of healthy cooking fat such as lard, ghee or coconut oil in a large stockpot set over high heat.
Add the meat in a single layer, making sure that the pieces do not touch, and cook until brown on all sides. Work in several batches if you have to so you don’t overcrowd the pot.
Once all the meat has been cooked, lower the heat to medium, add a little more fat to the pot if necessary then throw in the onion and garlic and cook, stirring often, until the onion is fragrant and becomes translucent.
Add the broth, red wine vinegar, Dijon Mustard, canned tomatoes and spices and bring to a simmer. Turn the heat down, cover and slowly simmer for 2½ to 3 hours, until the meat is really tender and practically all the liquid has evaporated.
Add water, carrots, celery and mushroom, bring to a boil then reduce heat and continue cooking until the vegetables are tender, about 30 minutes. Kill the heat, throw in the spinach, stir delicately and let the soup sit until the spinach is completely wilted, about 5 minutes.
Serve piping hot with a lot of freshly cracked pepper.