Place the slices of fennel in the middle of a rimmed baking sheet and drizzle with the olive oil. Place the fillet of salmon skin-side down on top of the bed of fennel and sprinkle it with the salt. Spread the mustard evenly over the top of the salmon and sprinkle with the fennel and coriander seeds. Press gently to help the seeds adhere.
Roast until a fork inserts easily in the thickest part of the fish and the flesh flakes, approximately 1 hour. Remove the salmon to a platter. Sprinkle with the fennel fronds and drizzle with olive oil.