Place in a mesh strainer and press to remove as much liquid as possible.
Using a food processor (or blender) puree cauliflower. Stir in cream cheese, onion powder, butter and 3/4 cup cheddar cheese. Set aside.
Shepherd's Pie
Preheat oven to 350°F.
Cook beef, onion and garlic until no pink remains. Drain any fat.
Add mushrooms, celery, carrot, tomato paste, Worcestershire sauce, and beef stock. Stir and simmer uncovered about 5-7 minutes or until most of the liquid is evaporated.
Place meat mixture in the bottom of a 2 qt casserole dish. Top with mashed cauliflower and finally remaining cheese.
Bake 25-30 minutes or until golden and bubbly. Rest 10 minutes before serving.