Cook the pasta: In a large pot, heat the milk over medium heat. Add the pasta, stirring frequently to avoid clumping. Do not boil the milk. Cook the pasta in the milk over medium heat for 15 to 20 minutes until pasta is al dente. Milk should be simmering, but not boiling, you don't want to burn the milk or cause it to separate. Reserve 2 cups of the milk before straining, set pasta aside.
Prepare the sauce: In a large, deep sided saucepan or braiser, melt the butter over medium-low heat. Whisk in the flour, continuously whisking for 2 minutes until the roux (flour-butter mixture) is a golden color. Reduce heat to low and slowly pour in the reserved milk while whisking. It will bubble and steam. Continue whisking until completely incorporated and there are no flour clumps. Repeat with the cream.
Get cheesy: Add the cheese one handful at a time, whisking into the sauce until incorporated. Once all the cheese is melted and smooth, stir in mustard, pepper, and 1 teaspoon salt. Add the pasta, mixing until all the pasta is well coated with the cheese sauce.
Prep the tuna: Remove the tuna fillets from the oil. Break apart into bite sized pieces over the macaroni and cheese. Gently stir into the pasta until well distributed. Spread the tuna casserole into an even layer and chill in the refrigerator for at least 4 hours until completely cold, overnight is best. This step is crucial in shaping the fritters.
Form and bread the fritters: Using a large cookie scoop or eye balling it with a spoon, scoop a ball of tuna casserole, pack together and roll slightly to smooth. You want each ball to be smaller than a baseball but a little larger than a golf ball. Repeat until all the tuna casserole is formed into fritter balls. Place the well beaten eggs in one bowl and the panko crumbs in another. Season the panko crumbs with ½ teaspoon of salt. Roll each tuna casserole ball in the egg wash, drain off the excess, then roll through the panko crumbs packing the crumbs on slightly. Shake of excess and set aside. Repeat with all the tuna casserole balls.
Fry: In a heavy bottom, deep sided pan such as a dutch oven, heat the oil to 350°F to 375°F. Depending on the size of your pan, fry 4 to 6 balls at a time for 3 minutes each side, until golden brown. After flipping the tuna mac and cheese balls, gently move them around in the pan to avoid sticking to the bottom. After frying, drain with a slotted spoon and transfer to a paper towel lined platter. Test one to ensure the center is hot and creamy.
Dipping Sauce: Combine the mayo, sriracha and garlic in a bowl. Serve with in a bowl for dipping or to drizzle over the top!