In a small saucepan, heat ½ cup grapefruit juice over medium heat. In a small bowl, whisk together cornstarch and remaining ¼ cup grapefruit juice. Add warm grapefruit juice; return to saucepan, stirring until thickened. Remove from heat, and stir in honey.
Place grapefruit slices on prepared pan, and brush with olive oil. Place fillets on top of grapefruit. Sprinkle fish with 1 teaspoon salt.
Bake for 10 minutes. Brush tops of fish with glaze, and bake 5 to 7 minutes more. Brush with glaze again.
In a small bowl, combine cubed grapefruit, vegetable oil, and remaining ⅛ teaspoon salt. Spoon on top of fish. Garnish with cilantro, if desired.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/grapefruit-baked-halibut-recipe/