This is a traditional Andalucian dish.
Peel and crush the garlic, sprinkle with olive oil and salt and let it infuse for 5 minutes. Peel and cut the onions into julienne style strips. Cut the tuna into chunks and season with salt.
Heat olive oil in a pan. Once the oil is hot, add the tuna and fry until brown on each side. This will be about 1-2 min on each side. Take out of the pan and set aside.
Now fry the onions and the garlic until the garlic is golden and the onions are translucent and soft. Add the sherry vinegar and let it simmer, then take the pan of the heat.
Add the paprika and oregano and stir into the onion garlic mixture. Add some water (about 50ml) and the tuna and put the pan back on the hob and cook until the water evaporates.
Season with salt and black pepper, serve and enjoy.