Preheat the oven to 350°F. Lightly grease eight cups of a standard muffin pan with avocado or olive oil.
In a large bowl, combine the ground beef, onion, Dijon mustard, sage and ½ teaspoon sea salt. Mix gently until everything is evenly incorporated, being careful not to overmix.
Divide the meat mixture evenly among the eight prepared muffin cups. Spoon a small amount of tomato sauce over the top of each muffin.
Bake for approximately 20 minutes, or until the centers reach an internal temperature of 160°F.
While the meatloaf muffins are baking, steam the cauliflower for 8–10 minutes, or until very tender.
Transfer the cauliflower to a food processor or high-speed blender. Add the coconut milk and remaining ½ teaspoon sea salt. Blend until smooth and creamy. An immersion blender may also be used.
Serve two meatloaf muffins per person. Spoon the mashed cauliflower over the muffins or serve it on the side. Drizzle with Primal Kitchen barbecue sauce, if desired.