Certain Sushi require the searing of the fish on the outside. Does this process require a Jew light the torch used to do the searing or is it not required? I have seen that some stores do and some do not. what is the opinion of the Big national Hecsherim or any modern day Poskim you are aware of and why sources would be appreciated?
Per the cRc it does not require Bishul Yisroel
Generally speaking, if a non-Jew cooks a food in a country wherein that food is eaten raw, then that food remains permitted to the kosher consumer regardless of where it is eaten. This is because food that is eaten raw is precluded from being bishul akum and, subsequently, the non-Jew's cooking is not considered to be a meaningful act. Thus the food was effectively not cooked by a non-Jew and is not, and can never become, bishul akum
Per the KOR Detroit since tuna can be eaten in its raw form, e.g. sushi, it is exempt from Bishul Yisroel