We're big fans of the way citrus pairs with creamy avocado. This colorful salsa would be equally at home with chicken.
Make salsa: Combine grapefruit, avocado, onion, cilantro, jalapeno, and lime juice in a medium bowl. Season with salt and set aside.
Heat oil in a large skillet, preferably cast-iron, over medium-high heat. Season halibut generously with salt and pepper and sear, undisturbed, until golden brown, 2 to 3 minutes. Flip and continue cooking until golden and flaky, 2 to 3 minutes more. Remove and let cool slightly, then flake with a fork.
Toast tortillas over a gas burner, turning, until lightly charred, about 1 minute. Top with fish, salsa, and cabbage.