How Best To Cut Frozen Tuna Before Dethawing? (Or Other Advice?)

This may be an odd question. I recently went to my local fishmonger, who had frozen blocks of tuna, and I was tempted to make poke with them, defrosting the fish in the morning to eat when I got back from work.

I got 4 blocks of fish of slightly varying sizes, totaling around ~1270g of fish (plus packaging). My hope was that I could cut the larger pieces of the frozen tuna into smaller blocks that could give me 5 portions instead of 4, as I think 250g of tuna per meal should be plenty. However, I'm not sure how best to cut the frozen fish because it's so hard. Any suggestions?