Niçoise On A Roll

Ingredients

• 1 crusty sourdough or other sandwich roll (3 to 4 oz.)
• About 1 tablespoon extra-virgin olive oil
• 1 thin slice red onion
• 3 thin slices firm-ripe tomato
• 6 thin slices cucumber
• 4 thin slices hard-cooked large egg
• 3 ounces (1/3 cup) drained canned tuna (in oil)
• 5 pitted nicoise olives
• 2 or 3 drained canned anchovies
• 2 to 3 teaspoons white wine vinegar
• Salt and pepper