- In a large pot of salted boiling water, cook the potatoes with the garlic until the potatoes are tender. Drain.
- With a potato masher, crush the potatoes and garlic with the butter. With an electric mixer, purée the potatoes with the milk until smooth. Season with salt and pepper. Set aside.
- With the rack in the middle position, preheat the oven to 200 °C (400 °F).
- In a skillet, brown the onions in the oil. Add the pepper and garlic and cook for about 2 minutes or until just tender. Season with salt and pepper. Add the tomatoes and cook for about 5 minutes. Remove from the heat. Add the tuna and olives. Adjust the seasoning.
- Pour into a 33 x 23-cm (13 X 9-inch) baking dish. Cover with the mashed potatoes and sprinkle with the paprika.
- Bake for about 30 minutes or until the potatoes begin to brown slightly. Finish cooking under the broiler, if desired.