Defrost the frozen tuna until it's just soft enough to cut through (see notes).
Stabilize your cutting board with a damp towel underneath, and grab a large sharp knife.
Use the knife to cut thin slices of tuna across the grain of the fish. Arrange them on the serving platter as you cut each piece, and season with salt and pepper as you layer the pieces.
Top the tuna with the capers, red onion, and more salt and pepper (if needed). Drizzle generously with olive oil, and sprinkle with lime zest. Add lime juice just before serving.
Serve with crusty bread or little toasts, or dressed salad greens.
Defrosting the Tuna:The tuna will be much easier to cut through if it's still slightly frozen, so you don't want it to defrost entirely before you use it.If you're going to defrost it on your counter (not necessarily the safest method), do not leave it out for more than 30 minutes. A safer way to defrost your tuna is to move it from the freezer into your fridge about 10-12 hours before you need to use it. The exact time it takes to defrost will depend on the size of the tuna and the temperature of your fridge.An Alternative to Cutting Thin Slices:If your knife skills aren’t great, or you’re worried about cutting yourself, you can cut the slightly defrosted tuna into 1 inch pieces, cover them with plastic wrap, and pound flat with a mallet.If You Don't Like Raw Fish:If you’re not a fan of raw fish, you can add the lime juice and let the fish sit for about 5-10 minutes. The acid in the lime juice will "cook" the fish, changing the colour, flavour, and texture. The longer the fish sits in the lime juice, the more "cooked" it will become. However, this usually results in less flavour from the fish, and a weird chewy texture. I recommend adding the lime juice just before serving for the best flavour and texture.To Reduce the Bite of Raw Onions:
rinse diced onions under running water (in a mesh sieve)
soak them in ice water for at least 10 minutes
Or soak them in lime juice with a pinch of salt for a quick pickled effect
Please read the blog post for detailed tips and explanations.