In a large cast-iron skillet, melt the butter over medium-high heat.
Add garlic and onions, and cook until slightly brown.
Place the trout, skin-side-down, in the skillet.
Add the sweet peppers, tomatoes, squeesed grapefruit juice and Four Peaks Grapefruit Radler.
Cook the trout, spooning the butter from the bottom of the skillet over the fish often, until just cooked through, about 5 minutes. Don’t flip the fish.
Transfer the fish to a serving plate.
Remove the skillet from the heat and scrape up any browned bits from the bottom of the pan. Spoon the pan sauce over the fish. Top with the cilantro and garnish with grapefruit slices.