In a frying pan, heat the oil and melt the butter over medium-high heat. Sear the fish fillets on each side, season, set aside and keep warm.
In the same pan, brown the shallot in the remaining fat. Deglaze the pan with the grapefruit juice and reduce the volume by a quarter. Add the maple syrup and bring to a boil.
Meanwhile, mix the flour with the butter to make a smooth paste. Add it to the sauce and whisk over simmering heat until the sauce thickens.
Place the fillets in the sauce and finish cooking the fillets in the sauce until it nicely coats the fillets.
Garnish with the grapefruit segments and scattered bits of chive.