Slice loaf in half lengthwise. Scoop out about 3/4 of the bread from the top and bottom halves to create pockets; discard scooped-out bread.
In a small bowl, whisk together the mayonnaise, garlic, and lemon peel. Spread generously over inside of loaf's top half.
In a medium bowl, stir together the tuna, tarragon, vinegar, and salt and pepper to taste. Spoon into bottom half of loaf.
Top tuna mixture with radish slices, then the tomato, eggs, and avocado slices. Spoon olives over all and top with the other loaf half. To serve, slice into 6 equal portions.
Note: Nutritional analysis is per serving.
Wine pairing: A fresh, lively dry rosé--preferably a Grenache--is great with this sandwich; licorice in the wine can echo the tarragon in the sandwich.