Make the Medium-Boiled Eggs: Bring a pot of water to a full boil. Gently lower the eggs into the water and cook for 8 minutes.
Transfer immediately to a bowl of ice water and let cool completely. Peel and slice. The yolks should be soft and slightly jammy. For slightly firmer yolks, cook 1 additional minute.
Make the Quick Pickle the Onion: In a small bowl, combine sliced red onion, red wine vinegar, a pinch of sugar, and a pinch of salt. Toss well and let sit for at least 20 minutes while preparing the remaining components. Drain before using.
Make the Olive Herb Salad: In a bowl, combine the chopped Castelvetrano olives, Kalamata olives, capers, parsley, basil and drizzle with olive oil and red wine vinegar, then toss to combine. The mixture should be rustic and spoonable, not finely minced.
Make the Anchovy-Dijon Spread: In a small bowl, stir together mayonnaise, chopped anchovies, and Dijon mustard until smooth and well combined.
Prepare the Bread: Using your hands, gently remove some of the soft interior of the baguette to create space for the filling. Spread the anchovy-Dijon mixture evenly on both cut sides of the bread.
Assemble the Sandwich: Layer the ingredients in the following order: Flaked tuna (gently break apart, do not mash), tomato slices, olive herb salad, sliced medium-boiled eggs, drained pickled red onions and finish with sliced pepperoncini.
Top with remaining bread half and press gently.
Rest and Serve: For best flavor, wrap tightly inparchment and let rest for 10 to 15 minutes before slicing. This allows the flavors to meld and slightly soften the bread. Slice and serve.