Preheat your oven to 180C/ 350 C and prepare a medium-sized baking dish. Add the oil to a nonstick pan over medium heat and brown the beef for 5-7 minutes, until browned. Finely dice the onion and mince the garlic, then add them to the meat and cook for 2-3 minutes.
Take the beef off the heat and set it aside. Finely chop the tomatoes and peppers and add them to the casserole. Roughly chop the cauliflower and add it to the baking dish along with the salsa.
Add the beef to the casserole dish. Add the cumin, paprika, cayenne, and salt. Mix to incorporate.
Transfer to the oven and bake for 20-25 minutes, or until the cauliflower has cooked all the way through. Take the casserole out of the oven and let it cool slightly. Meanwhile, finely shred the lettuce, slice the lemon, and cut the avocado into cubes.
To serve, add 1 cup of shredded lettuce to a plate, and top it with ¼ of the casserole. Add avocado and lemon slices to the plate. Serve immediately.