Process the rocket and garlic in a food processor until roughly chopped. Add the breadcrumbs and pulse until combined, then stir in the oil. Season with salt and pepper. Cover and chill until needed.
Put the mussels in a large saucepan with a tight-fitting lid and add water to a depth of 2.5 cm (1 inch). Cover and bring to the boil over a high heat. Cook the mussels, shaking the pan frequently, for 2–3 minutes, or until the shells have opened. Drain, discarding any that remain closed. Pull away and discard the empty shell halves, reserving only the halves with the mussels attached.
Place the mussels, flesh-side up, on a baking sheet. Divide the breadcrumb topping between the mussels and cook on the top shelf of a preheated high grill for 1–2 minutes until the breadcrumbs are golden. Serve immediately with lemon wedges on the side.