Line slow cooker with parchment paper. Lay a few lemon slices on the bottom with the fillets on top of those with the skin side down. Put pats of butter on top of each one. Pour broth inside and then season the top with salt and pepper.
Place additional lemon slices on top of the fillets if desired. Cover with the lid and cook on low for 2 hours or until the thickest part reaches the "doneness" you desire. It should be flaky and no longer translucent in the middle.
Lift out with a slotted spoon and serve.
Video
Notes
Adding Vegetables (option)
If you wanted to add some vegetables you could. I would add them underneath the parchment paper with an additional cup of broth with them to cook to tender. I would stick with baby carrots or large chunks of potatoes as they will cook at the same rate so it is all done at once.