In a large skillet (a 10-inch diameter works well), heat the olive oil over medium heat. Add the garlic and sauté 1 minute or until fragrant but not turning brown. Add the anchovies, mashing them with the back of a spoon. (They will dissolve into a paste and flavor the sauce with no obvious pieces.)
Add the tomatoes, capers, salt, pepper, and sugar. As the tomatoes cook and soften, mash them with a fork to help them form a chunky, rustic sauce. Simmer this mixture for 5 minutes.
Add the tuna steaks and cook for 3-4 minutes or until cooked to your liking, flipping to finish off the second side, about 30 seconds to a minute more. (We like our tuna a little pink inside. Cook longer for well done, shorter for rare. You can typically gauge how “cooked through” the steaks are by looking at the sides. It is also fine to cut into the center and take a peek to be sure.)
Sprinkle with feta and torn basil, if using. Serve with crusty bread, over pasta, or as is.