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