Preheat your panini grill or a grill pan over medium heat.
Break up the corned beef with your fingers and place into a bowl. Add a tablespoon of water to the top of the corned beef then cover with a paper towel. Place into the microwave and heat for 30 seconds. Place a little bit of mayonnaise on top of one slice of bread. Add the corned beef to the top followed by a piece (or two) of Swiss cheese. Spread some grainy mustard on another slice of rye bread then place it on top.
Place into your panini pan (or grill pan). Cook for 5 minutes or until crispy and golden brown. Serve immediately. Enjoy!
Side Note: I always cook my paninis dry. I find that using oil or butter on the bread makes the sandwich too greasy. Try it - I bet you will like it and it's way better for you!