Preheat the oven to 350°F (175°C). Grease a 9x13 inch (23x33 cm) casserole dish and set aside.

Steam the broccoli: Lightly steam the broccoli florets for 3-4 minutes until they are bright green and slightly tender. Drain and set aside.

Prepare the chicken: Season the diced chicken breasts with salt and pepper. In a large skillet, heat 1 teaspoon of olive oil over medium heat. Add the chicken and cook for 5-7 minutes, until browned and cooked through. Remove from the skillet and set aside.

Sauté the vegetables: In the same skillet, add the diced onions and cook for 3-4 minutes until translucent. Add the minced garlic and cook for another minute. Then, add the cauliflower rice and sauté everything together for about 5 minutes, until the cauliflower is tender. Set aside.

Prepare the creamy sauce: In a medium bowl, whisk together the Greek yogurt, skim milk, and the instant cream of mushroom soup mix until smooth and creamy.

Layer the casserole: Start by spreading the sautéed cauliflower rice and onion mixture at the bottom of the greased casserole dish. Next, add a layer of the cooked chicken pieces. Then, spread the steamed broccoli on top of the chicken. Finally, pour the creamy sauce evenly over the entire dish, ensuring all layers are covered.

Top with cheese: Sprinkle the shredded mozzarella cheese over the top of the casserole. For extra flavor, you can also add the grated Parmesan cheese if you choose to use it.

Bake: Cover the casserole with aluminum foil and bake for 20 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly, and the top is golden.

Serve: Let the casserole cool for about 5 minutes before serving. This dish is best enjoyed warm and can be stored in the refrigerator for up to 3 days.