In a large skillet, over medium heat, add the ground beef (or turkey) and diced onion. Cook until the meat is browned and the onion is soft, about 7-10 minutes. Drain excess fat if necessary.
Stir in the minced garlic and taco seasoning, followed by the drained diced tomatoes. Mix well and let simmer for about 5 minutes.
In a large mixing bowl, combine the cauliflower rice, half of the shredded cheddar cheese, half of the shredded mozzarella cheese, and the two eggs. Mix until all ingredients are well incorporated.
Season the cauliflower mixture with salt and pepper to taste.
Grease a 9x13-inch baking dish. Begin layering by spreading the cauliflower rice mixture evenly on the bottom of the dish.
Add the seasoned meat mixture on top of the cauliflower layer and spread it out evenly.
Top with diced bell pepper and jalapeño slices if using.
Finish by sprinkling the remaining cheese on top. Bake in the preheated oven for 25-30 minutes or until the cheese is bubbly and golden brown.
Remove from the oven and let sit for a few minutes before slicing.
Garnish with fresh cilantro and serve with a dollop of sour cream and slices of avocado if desired.