Preheat oven to 375 degrees and spray deep casserole dish with cooking spray.
Cut cauliflower into quarters and steam until tender. Add to food processor with yogurt or sour cream, cheese and salt. Blend until smooth. Set aside.
In a large pan, cook beef, onions and garlic until beef is no longer pink. Remove from pan and set aside.
Add vegetables to pan and cook until tender-crisp.
Mix beef mixture back in with vegetables and add remaining ingredients. Simmer for about 5 minutes until thickened and flavors combine.
Pour beef mixture in prepared casserole dish. Spread cauliflower mash on top. Add extra cheese on top, if desired.
Bake in oven for about 20 minutes until filling is bubbling and mash is slightly brown on top.