How to Make It
Preheat the oven to 350°. Set the garlic on a piece of foil, cut sides up. Brush with oil and wrap the garlic in the foil. Bake for 40 minutes, until softened; let cool slightly.
Meanwhile, light a grill. Brush the bread slices on 1 side with 2 tablespoons of the oil. Grill over high heat until lightly charred, about 1 minute per side.
Squeeze the roasted garlic into a bowl and mash with a fork. Stir in the remaining 2 tablespoons of oil and season with salt.
Spread the roasted-garlic puree on the grilled bread and arrange a few arugula leaves on top. Top with the mackerel and onion and tomato slices and serve.