Layer potato slices and fennel in a 6-quart oval slow cooker and pour hot broth over them. Cover and cook on high for 1 1/2 to 2 hours until potatoes are nearly tender.
Brush salmon with oil and season with salt and pepper. Arrange fillets in one layer over the hot vegetables and place lemon slices on top.
Cover and cook on high for 20 to 35 minutes until the salmon reaches 145°F in the thickest part and the potatoes are fully tender; check the fish after 20 minutes.
Lift fillets carefully with a wide spatula. Divide potatoes and fennel among 4 plates.
Place one salmon fillet on each serving and spoon over a little cooking broth.
Sprinkle each with one-quarter of the chopped dill and serve promptly.