Sprinkle salmon with salt and pepper: Be generous! Sprinkle on any other spices you’re using and rub them in with your fingers.
Line the slow cooker: Cut a large square of parchment or aluminum foil and press it into the slow cooker. This makes it easier to lift the delicate salmon out of the slow cooker later.
Place aromatics over the bottom of the slow cooker: If you’re using them, place a layer of lemon slices and sliced aromatics on the bottom of the slow cooker. This adds flavor, but isn’t strictly necessary.
Place one layer of salmon in the slow cooker, skin-side down. Top with more slices of lemon and aromatics, if using.
Add another layer, if needed: If you’re cooking more salmon than fits in a single layer, you can add a second layer. Place a piece of parchment over the first layer, lay the rest of the salmon over, and top with aromatics. (I don’t recommend adding a third layer.)
Choose your cooking liquid: The liquid helps to poach the salmon gently. It can be as simple as plain water, or as complex as a cup of amber beer with soy sauce and fish sauce mixed in. My standby is half water and half white wine. You’ll need between 1 and 1 1/2 cups of liquid.
Pour the liquid over the salmon: If cooking one fillet, add enough water to just barely cover. If cooking two layers, add enough water to come partway up the side of the top fillet.
Cook on LOW for 1 to 2 hours: The exact cooking time will vary based on your particular slow cooker, the number and thickness of your fillets, and how “done” you like your salmon. Check the salmon after an hour and continue checking every 20 minutes until it’s done. If you prefer fully cooked salmon, check it with a thermometer in the thickest part — the fish is done when it reaches 145°F.
Remove from the slow cooker: Lift the salmon from the slow cooker using the parchment or aluminum foil. Tilt the paper slightly as you lift to drain off the liquid.
Serve immediately, or cool and refrigerate for 3 to 4 days. Store leftover salmon in an airtight container.