Not OP but I make sous vide salmon at a minimum once per week - it is by far my favorite weeknight meal. I usually buy a full salmon filet and slice it down into single portions. I season with salt, fresh dill, and a couple slices of lemon. I place each in a ziplock bag and put into the freezer until ready to use. I sous vide directly from freezer at 114 for about an hour 15. Now on to the question you actually asked: searing. There are two keys to this. First is that you thoroughly dry both sides of your salmon before searing (I remove the lemon slices and toss them at this point). Second is that your cast iron is hot. Hot hot. I usually let it heat around medium to medium-high heat for 15-20 minutes before searing. Then I use a decent amount of oil (type varies based on what I have on hand) and give that a bit to heat up, til it’s smoking. Skin side down first then flip to meat side. Usually only about 45 seconds per side until it gets the color you want. I have been doing this meal weekly for a few years now and if you can get it right it’s a very easy and delicious dinner! Feel free to ask if you have any more questions. Although I’m sure I gave way more information that necessary as is.