How to Make It
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.