Preheat the oven to 75°C. Heat the serving plates.
Heat the butter in a large nonstick frying pan over medium heat. Sprinkle one side of the beet slices with cumin. Place the slices, seasoned side down in the frying pan and cook until the slices are heated mostly through, about 2 minutes. Season the top of the beet slices with cumin and turn over to continue heating all the way through.
Remove the slices and arrange in a circle with overlapping edges on the heated plates. Set aside and keep warm.
Place the coriander seeds and sprigs and coarse salt in a large, shallow saucepan of water. Bring to a boil. Reduce heat to simmer and allow the water to infuse for a few minutes.
Place the orange and lemon juices in a small saucepan and bring to a boil. Reduce heat to simmer. Reduce the juices to a syrup. Whisk in the grapeseed oil. Set aside and keep warm.
Poach the fish fillets until cooked, about 3 or 4 minutes. Drain well. Place the fillets on top of the beet slices and spoon the sauce over the fillets. Serve immediately.