Split roll in half horizontally; pull some of the soft interior out of top to accommodate filling. Drizzle cut surfaces with oil. On cut side of roll bottom, layer onion, tomato, cucumber, egg, tuna, olives, and anchovies. Drizzle evenly with vinegar, and sprinkle with salt and pepper to taste. Set roll top in place, cut side down, and press down gently.
Nutritional analysis per sandwich.