Slice cabbage into 5mm (1/4in) ribbons. Bring 1cm (1/2in) salted water to the boil in a large saucepan. Add cabbage and cook, covered for 4 minutes or until cabbage is no longer crunchy (think al dente like cooked pasta). Drain well and divide between two bowls.
If capers are packed in salt rinse them. Return the pan to the heat and reduce the burner to medium-high. Add a very generous glug of extra virgin olive oil and add capers. Fry for a minute or two until capers are crisping up. Remove capers from the pan and drain on paper towel.
Add tuna and the liquid from the can, passata and butter to the pot. Simmer for 5-10 minutes or until the sauce is reduced a little. Taste and season if needed but it probably will be fine with the salt from the tuna can. If it tastes too acidic or ‘not quite right’ melt some more butter in (more butter fixes everything!)