Cut the salmon into large pieces roughly the same size of your slow cooker. You can also cut them into smaller, individual-serving fillets.  

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 with 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.  

If you're using them, place a layer of lemon slices and sliced aromatics (onion, fennel, etc.) 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.  

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 broth with soy sauce and fish sauce mixed in. You'll need between 1 and 1 1/2 cups of liquid.  

Pour the liquid over the salmon. Add enough to just barely cover.  

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.  

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.