Remove the tops of the round zucchinis and hollow them out with a spoon, leaving about 1 cm of flesh around the edge (see our article on How to stuff round zucchini).
Peel and chop the onion, then sauté it in a bit of oil. Add the tomato purée, and season with salt and pepper.
If you’ve scooped out some nice flesh from the zucchinis (sometimes there are too many seeds), chop it up and add it to your mixture.
Add the flaked tuna (but not too much, it’s better with some chunks) and the grated cheese. You can also add a few black olives, as they pair well with tuna.
Stuff the zucchinis with this mixture. Place them in an oven dish, sprinkle with breadcrumbs, and place the zucchini tops next to them.
Bake for 30 minutes at 200°C (392°F)
Serve the zucchinis with their tops on the side for a prettier presentation!