Prepare the Meat Layer. Brown the ground beef in a skillet until fully cooked. Drain excess fat. Add the onion, bell pepper, and garlic to the skillet. Sauté until the vegetables are tender. Stir in the taco seasoning and a splash of water. Allow the mixture to simmer for a few minutes until the flavors meld.
Assemble the Casserole. In a greased 9x13-inch baking dish, spread the cooked meat mixture evenly as the first layer. Spread the riced cauliflower evenly over the meat layer. Layer the diced zucchini on top of the cauliflower. Scatter the diced tomatoes over the zucchini.
Add the Cheese Layer. Sprinkle the shredded cheddar cheese over the vegetable layer, creating a generous and even covering.
Bake and Serve. Place the casserole in the preheated oven and bake for 20-25 minutes, or until the cheese is melted and bubbly, and the casserole is heated through. Once baked, remove the casserole from the oven and let it rest for a few minutes before slicing. Garnish the casserole with black olives, cilantro, and green onions.