4 salmon fillets (5 to 6 ounces; 140 to 170g each)
2 tablespoons (30ml) extra-virgin olive oil
Aromatics such as fresh thyme, dill, parsley, thinly sliced shallots, and/or citrus zest (optional)
2 teaspoons (10ml) vegetable oil, if serving seared
Season salmon generously with salt on all sides.
Place salmon in a single layer in a gallon-size zipper-lock bag, or in batches in quart-size bags. Add olive oil to bag or divide it between the smaller bags. Add aromatics to bags, if using. Close bags, place in refrigerator, and let salmon rest for at least 30 minutes or up to overnight.
Using an immersion circulator, preheat a water bath according to the chart above and in notes section. Remove the air from the zipper-lock bags using the water displacement method: Seal bag almost all the way, leaving about an inch open. Slowly lower bag into water bath, holding the opened end above the water level. As bag is lowered, the water pressure should force air out of it. Just before it is totally submerged, seal bag completely. Use a rack, or clip bag to the side of cooking vessel using a binder clip, to prevent it from moving excessively. Cook 30 to 45 minutes for fillets 1 inch thick or less, or 45 minutes to an hour for fillets between 1 and 2 inches.
Carefully remove salmon from bags (it will be very delicate) and transfer to a double layer of paper towels. Discard aromatics and gently blot top of salmon with more paper towels.
To Serve Immediately: Carefully peel away skin and discard. Serve salmon immediately with the freshly skinned side down for better presentation.
To Serve Cold: Transfer salmon to refrigerator uncovered and allow it to chill completely. Store in a sealed container for up to a few days and serve cold in sandwiches or salads, stirred into pasta, or on top of rice bowls.
To Sear: Heat vegetable oil in a cast iron, carbon steel, or nonstick skillet over medium-high heat until shimmering. Carefully add salmon skin-side down and cook, pressing gently on fillets with the back of a flexible fish spatula to help maintain pan contact, until skin is browned and crisp, 1 to 1 1/2 minutes. Carefully flip and cook for an additional 15 seconds. Transfer to paper towels to blot off excess oil, then serve immediately.