How to Make It
Preheat the oven to 225°. Put the potatoes in a large pot of water and bring to a boil. Cook over moderate heat until tender when pierced, about 30 minutes. Drain and let cool slightly. Cut the potatoes into 1/2-inch wedges.
Meanwhile, in a small bowl, combine the anchovies with the minced garlic, parsley, crushed red pepper and 2 tablespoons of the vinegar. Whisk in 1/4 cup of the olive oil and season with salt and pepper.
Heat 1/4 cup of the oil in a very large, nonstick skillet. Add the chopped garlic and cook until softened. Add the potatoes, onion and tomatoes and cook over moderately high heat until warmed through, about 4 minutes. Add the remaining 1/4 cup of vinegar, season with salt and pepper and cook, stirring, for 1 minute. Transfer the vegetables to a large heatproof platter and keep warm in the oven.
Wipe out the skillet. Add 2 tablespoons of the olive oil and heat until shimmering. Season the tuna with salt and pepper and add half of the steaks to the skillet. Cook the tuna over high heat, turning once, until lightly browned but still pink inside, about 5 minutes. Transfer the tuna steaks to the platter. Repeat with the remaining 2 tablespoons of oil and tuna steaks. Drizzle the anchovy dressing all over the tuna and serve hot, warm or at room temperature.