1. Preheat the oven to 400°.
2. Put the unpeeled onions on a baking sheet and bake until the onions feel tender when pinched, about 1 hour. Remove from the oven and allow to cool.
3. Put the eggs and enough water to cover in a pot and place it over high heat. When the water begins to boil, reduce the heat to a simmer and cook the eggs for 10 minutes or less, depending on how dry you like the yolk. Place the pot under cold running water to cool the eggs and stop the cooking.
4. Peel the onions and cut into 1-inch chunks. Place them in a salad bowl.
5. Cut the tomatoes into wedges and add them to the onions
6. Put the anchovies, capers, vinegar, and salt in a food processor and pulse until almost creamy. Add the olive oil and run the processor just long enough to blend it into the dressing.
7. Peel the hard-boiled eggs, cut them into wedges, and add to the salad bowl.
8. Pour the dressing over the salad and toss gently. If adding the tuna, drain it and toss with the salad. Serve at room temperature.