In a large skillet heat the oil on medium heat. Add in the diced onion and garlic and cook for 3-4 minutes until the onion begins to soften. Add in the ground beef and use a wooden spoon to break the meat up into a fine crumble.
Cook the beef for 3 minutes before adding in the cumin, coriander, chili powder, chipotle powder and tomato paste. Continue to cook the beef until it begins to brown and then transfer it to a bowl and set aside, leaving any excess oil in the skillet.
Add the cauliflower rice to the skillet, season with salt and leave to cook for 5 minutes until the cauliflower softens but be sure not to overcook it.
While the cauliflower is cooking, make the sauce by combining in a blender or nutribullet the mayonnaise, lime juice, scallions and cilantro and blending until completely smooth.
To assemble divide the lettuce between two bowls. Top each with the ground beef, cauliflower rice, chopped red onion and tomatoes, avocado and drizzle with the sauce before serving.