Cut off top of peppers, remove seeds. Drop in boiling water (with a little salt).
Let stand for a few seconds and drain. Then set aside.
Drain tuna chunks. Flake into big pieces and add diced potatoes.
Melt butter in saucepan then add flour, salt and pepper. Stir until well blended. Then remove from heat.
Then combine tune and the above milk mixture. Fill pepper with this mixture.
Cover with bread crumbs and grated cheese.
Bake in a pre-heated oven at 340 °F for 20 minutes.