Preheat Oven: Preheat oven to 475 degrees.
Cook Cauliflower Rice: Cook frozen cauliflower rice in microwave, according to package instructions.
Cook Beef: While cauliflower rice cooks, in a large pan over medium-high heat, heat oil. Once hot, add onions and cook until fragrant, about 1 minute. Add ground beef and cook, breaking meat up as it cooks. Once beef is mostly cooked through, stir in garlic powder, Italian seasoning, red pepper flakes, salt, and pepper.
Add In And Wilt Spinach: Reduce stovetop heat to medium. In small batches to pan of cooked beef, add in spinach and cook until wilted, about 1 minute, stirring occasionally.
Prepare Casserole: To a 9x13-inch casserole dish, add in steamed cauliflower rice and sour cream and stir together until well combined. Add cooked beef and spinach mixture, stir again, then flatten into an even layer. Top dish with shredded cheddar.
Bake Casserole: Transfer casserole dish to oven and bake until cheese is golden and bubbling, about 13 minutes.
Serve: Remove dish from oven and allow to cool slightly before slicing and serving.