Preheat the oven to 375°F (190°C). Grease a 9-by-13-inch (23-by-33-cm) baking dish.
Place the bacon on a rimmed baking sheet, place in the oven and bake for 12 to 15 minutes, until cooked through. Roughly chop the bacon. Set aside.
Turn up the oven to 400°F (205°C). Poke holes in sweet potato with a fork. Place on a rimmed baking sheet and bake for 35 to 40 minutes or until the sweet potatoes are soft and easily pieced through with a knife. The time may range depending on the thickness of the sweet potatoes.
While the potatoes are baking, steam the cauliflower. Once cauliflower is fork tender, place it in a food processor or blender and puree until cauliflower becomes soft and resembles mashed potatoes. Then add the coconut milk and salt and pepper and continue to blend until you have a smooth consistency. Remove from the blender and set aside. Clean out the blender for the sweet potato mash.
When sweet potatoes are soft, remove their skins and place in the food processor or blender. Blend until the sweet potato breaks down, then add the coconut milk, sage, and salt and pepper and puree until smooth. Set aside for later.
Lastly, place a large pan or Dutch oven over medium-high heat. Add the ground beef and break it apart with a wooden spoon. Once the meat begins to brown, add the garlic, yellow onion and button mushrooms. Sprinkle with salt and pepper and continue to cook until the meat is cooked through, then remove the pan from heat.
Create the layers: Use a slotted spoon to remove half of the meat mixture from the excess liquid in the pan and place in the prepared baking dish in one layer. Then pour the cauliflower mash mixture on top of the meat and spread out. Then, using the slotted spoon, add the other half of the meat on top of the cauliflower mash. Lastly, pour the sweet potato mash on top and spread it out on top of the meat.
Turn oven down to 350°F (175°C) degrees. Place the casserole in the oven and bake for 30 minutes, or until the casserole begins to bubble. Then turn on the broiler to high and cook for 5 minutes to brown the top of the casserole. Let rest to thicken and garnish with the bacon before serving.