Heat the oil and gently fry the onion and garlic until golden brown. Mix the onion and garlic with the tuna and set aside.
Mix the potatoes and peppers in a flameproof casserole and just cover with warm water. Bring to the boil, cover and simmer for about 15 minutes, until the potatoes are almost cooked.
Add the tuna and onion mixture and the tomato sauce and season to taste with salt. Bring to the boil, cover and simmer for about 8 minutes until the tuna is cooked. Add the bread slices before removing from the heat and serve immediately.