How to Make It
In a medium bowl, combine the avocado slices with the jalapeño and the 1 teaspoon of marjoram; season with salt and pepper. Add the lemon juice and the 2 tablespoons of olive oil and toss gently.
Light a grill and brush the grates with oil. Brush the bread with olive oil and grill over high heat, turning once, until lightly charred in spots. Rub the bread with the cut sides of the garlic clove.
Brush the mackerel with olive oil and season with salt and pepper. Grill skin side down until lightly charred, about 3 minutes. Carefully flip the fish and grill until cooked through, about 2 minutes longer. Arrange the fish on the bread, skin side up. Top with the lardo and avocado salad. Garnish with marjoram leaves and serve right away.
Lardo is available at specialty food shops and Italian markets.