Preheat oven to 350 degrees.
Brown ground beef.
Add mushrooms, cauli rice, diced bell peppers and taco seasoning. Stir until combined and cook until completely cooked through. Stir occasionally. Add tomato paste and water. Stir and allow water to cook down. Taste and adjust as needed.
Add the beef mixture to a 9×13 dish. Top with shredded cheese. Bake in oven for 15 minutes. If you are cooking this cold from the fridge, you will need to bake it longer. Remove from the oven and carefully spread sour cream on top.
Sprinkle with whatever toppings you desire. Serve warm.