In a large bowl, combine the cabbage, onion and parsley. Drizzle with the lemon juice and 2 tablespoons olive oil; toss and season with salt and pepper. Refrigerate for at least 15 minutes. Toss with the grapefruit just before serving.
In a large nonstick skillet, heat the remaining 2 tablespoons olive oil over medium-high heat. Season the cod with salt and pepper. Add to the pan and cook, turning once, until golden, about 8 minutes. Serve with the slaw.