Make the adobo: pound the garlic, cumin, vinegar, and salt and mix them with the oil in a large flameproof earthenware dish or pan in which the fish fits. Finally, add the pimentón and bay leaf. Heat very gently for 8-10 minutes. Remove and cool. Cover and chill until required.
Remove the skin and from the tuna and cut it into slices (or ask your fishmonger to prepare the fish for you). Beat the adobo ingredients, except for the bay leaf, and put into a wide deep heavy-bottomed earthenware pot, like a cazuela, or a frying pan where the tuna will fit easily in a single layer.
Lay the tuna in the adobo, so it is completely covered - top up with a little more olive oil if you need more liquid - and cook very gently for about 8-10 minutes, depending on the thickness of the fish slices.
Take off the heat and leave to cool, cover and chill. If you can leave this for at least a day for the flavors to soak into the fish. Bring to room temperature to serve gently warmed through with potatoes, or rice, or at room temperature in the summer with plenty of good bread.
A note about the fish: the tuna can be any member of the tuna family. In the old days this would have been made with red tuna, or today bonito, but today lesser known small fish are used.