1) Preheat the oven by setting it at 450°.
2) Take a large sauce pan and melt butter in it.
3) Sauté after adding the onion and green pepper to it until they become soft.
4) Mix in the salt and flour and cook for a minute stirring all the while.
5) Add milk to the mixture gradually and cook over medium heat until it thickens and turns bubbly.
6) Add the lemon juice and tuna before stirring them in.
7) Take a greased casserole 1 ½ quart in size and add the tuna mixture with the aid of a spoon.
8) Spread the cheese roll on top of it.
9) Bake it by placing it in the pre-heated oven for half an hour or until the rolls turn brown.