First things first, slice that bagel in two. Toast until golden and crisp. We're talking that signature New York bite – nothing less.

Grab a bowl. Toss in your drained tuna, the chopped red onion, that classic mayonnaise, and a hit of mustard. Now, season it without fear with a good dash of salt and pepper – it's the New York way! Mix it all together until it's ready for its close-up.

Spread a generous layer of your tuna mixture onto one half of that perfectly toasted bagel.

Now for the main event: lay those cheese slices right on top of the tuna mixture. Pop that bagel half under a hot grill for 2-3 minutes, or until the cheese is gooey, melted, and bubbling to a golden brown.

If you're feeling fancy (and you should be), hit it with an optional sprinkle of fresh chives for that subtle green finish. Carefully top with the other toasted bagel half. Now, grab hold and devour it like you mean it. This ain't just a bagel; you just made it here, and you can make it anywhere!