Coat a large saute pan with olive oil and bring to a high heat. Coat the bottom of another smaller saute pan with olive oil. Sprinkle the fish with salt on both sides. When the large saute pan is screaming hot but not quite smoking, lay the fish fillets in the pan, skin-side down. Do not crowd the pan, you may have to work in batches. After you put the fish in the pan, place the other small saute pan directly on top of the fish fillets. This applies gentle pressure to the fish and forces the skin to have contact with the pan and will create crispy skin. Cook the fish for 3 to 4 minutes and then remove the top pan. Shake the pan a little to unstick the fish. Use a fish spatula and flip the fish fillets and cook for 2 more minutes on the other side. Remove from the pan and serve or keep warm until the remaining fish is cooked.
In a large mixing bowl, dress the greens and red onions with olive oil and grapefruit juice and season with salt. Toss in the olive slivers and grapefruit supremes.
Divide the salad among 4 serving plates and sprinkle with feta. Lean a fish fillet on each salad. Serve immediately.