Gather the ingredients. Preheat the oven to 200°C/400°F.
Cut the bell peppers in half vertically and remove the seeds and white membrane.
Gently boil the pepper shells in salted water for 5 minutes. Drain well.
In a mixing bowl, combine the onion, ½ of the bread crumbs, beaten egg, milk, salt and pepper, lemon juice, and tuna.
Fill the pepper shells with the bread crumb mixture.
Mix the melted butter with the remaining 1/2 of the bread crumbs; sprinkle over stuffed peppers. Place in a shallow baking dish and bake for 15 minutes. If desired, top with shredded cheese 5 minutes before the peppers are finished baking.