Vinaigrette is a simple blend of oil, vinegar, salt, and pepper. Mustard is also commonly added. You can add an extra flavoring or two, such as herbs, scallions, garlic, or cheese.
Soak 1/2 onion, thinly sliced, in very cold water for 20 minutes. Drain on paper towels. Pit and chop 1/2 cup olives (optional). Peel and slice 2 hard-cooked eggs.
Brush 8 thick slices country bread on one side, using one half recipe of garlic vinaigrette. Make sandwiches, layering romaine lettuce, two 6-ounce cans drained tuna, olives, onion, and eggs.