Make pickled cucumbers. Combine boiling water with rice vinegar, sugar, and salt, stirring until sugar is dissolved. Add cucumber and red onion and let sit for at least 30 minutes or overnight.

Prepare chili crisp. Combine chili flakes with ginger and sugar in a heat safe bowl and set aside.

Combine oil with shallots, garlic, star anise, and peppercorns in a medium saucepot. Bring to a simmer over medium heat. Reduce heat to low and continue to simmer until shallots are browned, about 7-8 minutes. Remove from heat. Place a strainer on top of the bowl with chili flakes and pour hot oil directly through strainer into the bowl. Discard peppercorns and star anise but reserve shallots and garlic. Let oil cool, then add back shallots and garlic.

Combine canned tuna with mayonnaise, celery, cilantro, scallions, chili crisp, yuzu, and dijon mustard, mixing until well combined. Season with salt and pepper.

Assemble sandwiches. Toast ciabatta and top bottom bun with yuzu tuna salad, pickled cucumbers and onion, arugula, avocado, and tomato. Drizzle extra chili crisp if deserved and top with remaining bun. Serve.