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.