In a bowl, mix smoked salmon, feta, and green onion. Cut a pocket into the side of the halibut fillet and season with salt & pepper. Stuff the salmon mixture into the halibut.
Spread 1 tbsp of butter on the bottom of an oven proof sauce pan and add the halibut and the fish stock. Top with red bell pepper and bake at 400 degrees for 10-12 minutes.
Remove the fish from the sauce pan, and bring the remaining liquid to a boil on the stove top. Add the remaining 2 tbsp butter, and simmer until thickened. Pour over the halibut and serve.