The reason why your tuna melt is soggy is a result of excess moisture in the tuna salad or insufficient crispness of the bread.
In order to prevent the tuna melt from getting soggy you need to ensure that you drain the tuna thoroughly and use toasted bread which is best as the toasted bread creates a barrier against the moisture.
To make a tuna melt place the tuna into a bowl with the spring onion, red onion and cheddar.
Then sandwich the filling between the two slices of bread, then butter the outsides of the sandwich.
Heat the oil in a frying pan over a low heat.
Then flip and repeat to cook the other side.
Tuna melts are basically a tuna salad sandwich that has melted cheese and is often grilled or pan fried and often includes bread, tuna mixed with mayonnaise and other ingredients such as onion, celery or pickles and a slice or a few slices of cheese which melts well.
Mayo is used in tuna melt as mayo or mayonnaise is common and essential as an ingredient in a tuna melt as the mayo binds the tuna with the other ingredients such as seasonings, onions, celery etc.