In a medium saucepan, heat the tallow or oil over medium-high heat. When the oil just begins to smoke, add the meat in large chunks. Turn up the heat to high and sear the meat without disturbing it until it browns.
Using a spatula, turn the pieces over to brown on the second side, then add the onion to the pan. Allow the onion to sauté for 4 to 5 minutes without disturbing it, then add the garlic and mushrooms.
Use a spatula to break up the chunks of meat and evenly disperse the mushrooms and garlic. Continue to sauté over high heat for 2 to 3 more minutes.
Stir and continue to cook until the beef is cooked through. Remove the beef mixture to a large bowl.
Add the cauliflower and sweet potato to the pan, along with the last 2 tablespoons of fat or oil. Sauté this mixture over high heat for 5 to 8 minutes—stirring once every minute until the vegetables are just cooked through.
Add the baby spinach and stir continuously until all of the spinach is just wilted and incorporated.
Transfer this mixture to the bowl with the beef, sprinkle with salt and pepper, to taste, and stir to combine. Serve with the hot sauce of your choice.