Chop potato into small pieces.
Place cauliflower rice and potatoes in a medium saucepan and cover with water. Bring to a boil and cook for 5-10 minutes, until potato is tender.
Drain. Add butter and salt. Mix with a hand mixer. Set aside.
Preheat oven to 400 degrees.
In an oven safe skillet, fry ground beef and cook until browned. Drain excess grease. Add onion, celery, and carrot. Cook for 5 minutes. (If using extra frozen vegetables, add now.)
Add garlic, salt, pepper, and paprika.
In a separate bowl, whisk together broth, apple cider vinegar, rosemary, and corn starch. Pour over the beef and simmer for 5 minutes until the sauce has reduced and thickens up.
Remove from heat, top with cauliflower topping.
Bake fore 20 minutes or until meat layer starts bubbling.
Cool for 10 minutes before serving.