Using a sharp knife, remove the peel and bitter white pith from the grapefruit. Working over a medium bowl, carefully cut in between the membranes to release the sections into the bowl. Squeeze the remaining grapefruit juice into a separate bowl.

Put the flour, egg, and cornmeal in 3 shallow bowls. Season the trout fillets with salt and pepper and lightly dust them with flour, tapping off the excess. Dip the fillets in the beaten egg, then dredge them in the cornmeal.

In a large skillet, melt 1 tablespoon of the butter. Add the sage leaves and cook over moderate heat, turning once, until crisp, about 2 minutes per side. Transfer the sage leaves to a plate lined with paper towels to drain. Add 1/4 inch of the vegetable oil to the skillet and heat until shimmering. Carefully add the trout fillets and fry over moderately high heat until golden brown and just cooked through, about 3 minutes per side. Divide the fillets among 4 plates.

Discard the oil and wipe out the skillet with paper towels. Add the fresh grapefruit juice and water to the skillet and bring to a boil; cook for 1 minute. Reduce the heat to moderate, add the grapefruit sections and simmer the sauce for 1 minute. Remove the skillet from the heat and swirl in the remaining 2 tablespoons of butter, 1 tablespoon at a time. Season with salt and pepper and spoon the sauce and grapefruit sections over the trout. Garnish each fillet with 2 sage leaves and serve immediately.