Heat oven to 200C/180C fan/gas 6. Brush each salmon fillet with half the agave syrup or honey, sprinkle on some black pepper, then press on the seeds. Transfer to a baking tray and bake for about 15 mins, depending on the thickness of the fish, until the fish is cooked through and the seeds crunchy.
Meanwhile, cut the peel from the satsumas and cut into segments, reserving all the juice that comes out for the dressing. Whisk the juice with the olive oil, vinegar and some seasoning in a small bowl.
Toss together the watercress, beetroot and satsuma segments with the dressing and divide between plates. Add a piece of salmon to each one.