Dice tomatoes, slice onions, crush or chop garlic and cut cabbage into desired sizes. Break tuna into desired sizes.
Heat oil in a saucepan on medium heat. Add in onions and garlic, frying till onions are translucent. Add diced tomatoes, pepper and rosemary and let it cook for about 5 minutes.
Add tuna and cabbage and simmer for 10 to 15 minutes. Taste and adjust for salt.