Preheat the oven to 375°F (190°C).
Bring a large pot of salted water to a boil. Add the chopped cauliflower florets and cook for 5 minutes until just tender. Drain well and set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook for 4-5 minutes until softened.
Add the minced garlic to the skillet and cook for 1 minute until fragrant. Remove from heat.
In a large mixing bowl, combine the cooked cauliflower, sautéed onion and garlic, drained tuna, shredded cheddar cheese, shredded mozzarella cheese, sour cream, mayonnaise, heavy cream, Dijon mustard, smoked paprika, black pepper, and salt. Mix well until fully combined.
Transfer the mixture to a greased 8×8-inch baking dish and spread evenly.
Sprinkle the grated Parmesan cheese evenly over the top.
Bake in the preheated oven for 20-25 minutes until the casserole is bubbly and the top is golden brown.
Remove from oven and let it rest for 5 minutes.
Garnish with chopped fresh parsley before serving.