Prepare the breadcrumbs. Put the bread in a clean, dry food processor and process into crumbs. Add the lemon rind, garlic, oil, parsley and chillies (if using) and mix well.
Heat a nonstick frying pan over a low heat and add the breadcrumb mixture. Toast gently for about 5 minutes, stirring frequently to prevent burning, until crisp and golden. Transfer the breadcrumbs to a plate lined with paper towels to absorb any excess oil and leave to cool.
Mix the dry mussels with the pesto sauce in a dry bowl until thoroughly coated, then divide between 2 large pieces of foil. Fold up the edges and scrunch them together to seal. Heat a griddle pan over a medium heat, put the foil parcels directly on the griddle and cook for 15–20 minutes until the mussels have opened. Discard any that remain closed.
Put the foil parcels in warm bowls. Diners open the parcels themselves and sprinkle the crispy breadcrumbs over the mussels and add more pesto to taste. Serve with cooked asparagus tips and plenty of crusty bread.