Bring a large pot of water to a boil. Cook the gluten-free rotini 2 minutes less than the package directions. Drain well.
Preheat the oven to 350°F.
In a large bowl, combine the tuna, mayonnaise, lemon zest, black pepper, lemon juice, Dijon mustard, green onions, parsley, tarragon, cheddar, and sour cream.
Stir the cooked pasta into the tuna mixture until everything is well combined.
Spoon the mixture into a small 2-quart casserole dish, approximately 7 x 12 inches.
Top evenly with the crushed cornflakes.
Bake for 25–30 minutes, or until the casserole is heated through and the topping is lightly crisp.
Garnish with additional fresh parsley and serve warm.