Bring a medium saucepan of water to a boil. Add the potatoes and cook until tender, about 20 minutes. Drain the potatoes and let cool slightly, then cut into quarters.
In a medium skillet, heat the 3 tablespoons of oil. Add the potatoes and cook over high heat until lightly browned in spots, about 5 minutes. Add the garlic and cook until the potatoes are browned and the garlic is crisp, about 3 minutes longer. Season with salt and pepper and keep warm.
Heat a grill pan. In a bowl, toss the tomatoes, capers and lemon juice; season with salt and pepper. Brush the mackerel with oil and season with salt and pepper. Grill over high heat for 7 minutes, turning once, until lightly charred and cooked through. Transfer the fish to plates and top with the salsa. Serve the potatoes alongside.
White wines work best with mackerel (which can make reds taste metallic). Try an Italian blend of Fiano and Greco grapes.