Heat fat over medium heat in sauce pan. Add onions and cook until onions start to brown. Stir in cauliflower and thyme sprigs.
Measure pan drippings and add enough stock to equal 2 cups of liquid. Add pan drippings/stock mixture to pan with vegetables. Simmer until cauliflower is fork tender, about 10 minutes. Remove herb stems.
Carefully transfer mixture to Vitamix. Blend on high until smooth and creamy.
Return gravy to pan to reheat. Add more stock to thin to desired consistency, if needed. Season with salt and pepper to taste.