Lay the trimmed asparagus in an even layer across the bottom of the slow cooker.
Set the salmon fillets on top of the asparagus.
Stir together the melted butter, minced garlic, lemon juice, chopped dill, salt, and pepper.
Pour the butter mixture evenly over the salmon.
Arrange the lemon slices across the top of the salmon.
Cover and cook until the salmon flakes easily and the asparagus is tender.
Remove the lemon slices before serving if you prefer.