Whip all the velouté ingredients. Marinate the tilapia fillets for 2 hours in the refrigerator. Preheat the oven to 400°F. Drain the tilapia and reserve it separately from the velouté. In a small saucepan, melt the butter and oil. Sear and brown the garlic for 3 minutes. Season with salt and pepper.
Deglaze with the velouté and reduce for 5 minutes. Place the tilapia fillets on a baking sheet and bake for 12 to 15 minutes. Meanwhile, pour the fish stock into the velouté mixture. Reduce for 5 to 7 minutes. Add the cream, then reduce and thicken for 5 minutes. Incorporate the manipulated butter by whisking vigorously and let the sauce thicken for 5 minutes. Adjust the seasoning. Serve each fillet on a bed of grapefruit velouté, decorated with pink grapefruit quarters.