Mix together the tuna, mayonnaise, mustard, ¼ cup cheese, onion, salt, pepper, and cayenne.
Transfer the mixture to a small, 1-quart casserole dish. Sprinkle the remaining ¼ cup shredded cheese on top.
Bake until hot, for about 15 minutes.
Sprinkle with parsley and serve immediately, with cut-up veggies and/or pork rinds for scooping out.
Video
Notes
You can bulk this casserole up by adding 2 cups of cooked riced cauliflower, increasing the mayonnaise to ½ cup, and dividing it into 3 servings. The leftovers keep well in the fridge for 3-4 days, or you can freeze them for up to three months if you want to keep them for longer. Gently reheat them in a 300°F oven or in the microwave at 50% power.