1. Spray the insert of a 6-quart slow cooker with cooking spray. Set the slow cooker to preheat on High while you assemble the recipe
2. Halve one of the lemons and thinly slice one half. Place the onion and lemon slices in an even layer over the bottom of the slow cooker.
3. Lay the salmon fillets, skin-side down, in one layer over the vegetables and tuck in the bay leaf. Sprinkle liberally with lemon pepper and squeeze the juice of the lemon half over the salmon.
4. Bring the broth to a simmer in a saucepan on the stovetop or in the microwave and gently pour it around the salmon—it should come up the sides of the fillets and not quite cover the top; add water if necessary. Cut a piece of parchment paper to the shape and size of your slow cooker so that it just fits inside. Lay it lightly on top of the fish, cover the slow cooker, and cook on High for 1 to 1 ½ hours, depending on the thickness of the fillets.
5. Use a flat spatula to remove the salmon and serve immediately or let cool completely (discard the bay leaf and vegetables). If serving immediately, cut the remaining lemon into wheels to garnish and serve with the fish, along with tartar sauce. Sprinkle with extra lemon pepper before serving, if desired.