Heat a panini grill, or a small frying pan over medium heat.

Next, open tuna can and drain the water into the sink.

Scoop the tuna into a bowl and add mayonaise, salt, and pepper. Mix well.

In a small pan, add olive oil. Add onion and saute until golden.

Prepare your hatch chiles. You can also use canned green chiles in the store if you prefer.

Butter one side of each slice of bread. Put two slices butter side down on the pan.

Add a slice of cheese to each slice of bread.

Next, add a layer of tuna fish.

Top the tuna with grilled onion and chiles.

Add a slice of cheese on top. Cover with the top piece of bread. Make sure the butter side of this slice faces upward.

Check the bottom slice of bread that is touching the heat. 

When this slice looks golden, flip the sandwich with a spatula to cook the other side.

This is ready when the cheese is all melted.