Spread 1 tbsp. of butter on one side of each slice of bread. Place them buttered side down on a cookie sheet.
In a small to medium mixing bowl, use a fork or spoon to thoroughly mix together the tuna, mayo and onions.
Spread the tuna mix over the non-buttered side of two slices of bread.
Then sprinkle with 1 tbsp. of cheese on each sandwich and close the sandwich with the last two slices of bread (butter side up).
Place the cookie sheet in the oven for 7-10 minutes. Then flip the sandwiches and bake for another 7-10 minutes. You want the bread to be golden brown when you flip it.
Remove from oven, allow to cool slightly, cut in half and serve.
Notes
Please note that the nutrition data given here is a ballpark figure. Exact data is not possible.