Cook the frozen cauliflower rice in the microwave, according to the package instructions.
Cook onions until fragrant. Add in ground beef and cook, breaking the meat up as it cooks. Once the beef is mostly cooked through, stir in spices. Don't drain the beef.
Reduce the stovetop heat then, in small batches, add in spinach and cook until that has wilted, stirring occasionally.
To a baking dish, add the steamed cauliflower rice and some sour cream and stir together. Add in the spinach and beef mixture, stir, and then flatten into an even layer. Sprinkle shredded cheddar cheese on top.
Transfer the casserole dish to the oven and bake at 475 degrees F until the cheese is golden and bubbling.
Remove the dish from the oven and allow it to cool slightly before serving.