Heat the oil in a large pot over medium heat. Add the garlic and cook, stirring often, until fragrant, 1 to 2 minutes.
Add the chard, tomatoes (and their juices), and 1 teaspoon each salt and pepper. Cook, stirring often, until the chard is tender, 8 to 10 minutes.
Nestle the fish in the liquid and cook, covered, until the fish is opaque throughout, 10 to 12 minutes.
Serve topped with the tapenade.