For the Cucumbers: Whisk together the vinegar, sugar, water, chilli and salt in a small saucepan. Bring to a boil over medium-high heat and stir until sugar dissolves, just a minute or two. Remove from heat and pour into a non-reactive airtight storage container. Wash and dry cucumbers then slice very thinly crosswise. If using the long English cucumber, you will need about 9 inches (23 cm) of it. (Save the rest for salad or snacking). Add cucumbers to the warm pickling mixture. Allow to cool, then cover and refrigerate at least 1 hour or up to overnight.

For the Sauce: Whisk together the soy sauce, lime juice, sugar and ginger in a small bowl. Sprinkle scallions on top and let sit while you prep and cook the tuna.

For the Tuna: Remove fish from refrigerator and set aside. Place both kinds of sesame seeds in a wide bowl and season with salt and pepper, tossing all ingredients together. Place the tuna steaks, once at a time, in the bowl and use fingers to press sesame mixture onto both broad sides of the fish filets.

Heat oil in a large nonstick saucepan over medium-high heat until shimmering. Add tuna steaks and sear for a minute or two or just until the sesame seeds begin to smell fragrant and take on a bit of color. Flip filets over and cook second side as for the first. You can use tongs to help sear the tuna steak’s sides, if you like. The fish will remain rare and pink in the center. Remove to a cutting board and allow to sit for a minute. Slice tuna against the grain about 1/4 inch (6 mm) thick into slabs. Fan slabs of fish out on plates, arrange a mound of pickled cucumbers alongside and drizzle with sauce. Offer extra dipping sauce in small bowls. Serve immediately.