How to Make It
Combine the tomatoes, onion, lemon juice, 1 tablespoon of the oil, and ¼ teaspoon each salt and pepper in a medium bowl. Let sit for 10 minutes.
Meanwhile, heat 2 tablespoons of the remaining oil in a large skillet over medium-high heat. Add the escarole, garlic, and ¼ teaspoon each salt and pepper and cook, tossing occasionally, until wilted and tender, 3 to 4 minutes. Let cool slightly, then add to the tomatoes and toss to combine.
Heat the remaining 2 teaspoons of oil in a large nonstick skillet over medium-high heat. Season the halibut with ¼ teaspoon each salt and pepper and cook until opaque throughout, 3 to 4 minutes per side.
Serve the tomato and escarole mixture topped with the fish.