Preheat your oven to 180°C (350°F). While the oven is heating, grease a large baking dish with a little oil or nonstick spray to ensure your casserole doesn't stick after baking. I always like to grease the dish generously, which makes serving and cleaning up much easier.

In a large mixing bowl, add the drained tuna and use a fork to flake it into smaller pieces. Stir in the mayonnaise, sour cream, Dijon mustard, salt, black pepper, smoked paprika, and chopped green onions. Mix everything together until you have a well-combined and smooth mixture. Then, gently fold in the cooked and chopped cauliflower and shredded mozzarella cheese until everything is evenly distributed.

Transfer the tuna mixture from the bowl into the prepared baking dish. Spread it out evenly so it bakes uniformly. Sprinkle the shredded gruyere cheese over the top, creating a nice layer that will melt and form a golden crust.

Place the assembled dish in the oven and bake for 15-20 minutes, or until the cheese topping is bubbling and golden brown. Keep an eye on the casserole toward the end of baking time to prevent the cheese from over-browning. I like to bake until the edges are slightly crispy for extra texture.

Remove the casserole from the oven and let it sit for a minute to cool slightly, which makes it easier to serve. Scoop into bowls or onto plates and enjoy while hot.