Preheat the oven to 375°F (190°C).
In a medium skillet, melt the butter over medium heat.
Add the chopped onion and sauté until translucent, about 4 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Reduce heat to low and stir in the heavy cream and cream cheese until the cream cheese is melted and the mixture is smooth.
Remove the skillet from heat and stir in the cheddar cheese, mozzarella cheese, Parmesan cheese, Dijon mustard, salt, black pepper, and smoked paprika until well combined.
Fold in the flaked tuna and peas until evenly distributed.
Transfer the mixture to a greased 8×8-inch baking dish and spread evenly.
Bake uncovered for 20 minutes, or until the casserole is bubbly and the top is lightly golden.
Remove from oven and sprinkle with chopped fresh parsley before serving.