Make a herb butter: finely chop 1 clove of garlic. Mash it to a soft paste with a little sea salt using a pestle and mortar. Finely chop 1 tbsp of thyme leaves, 10g of parsley leaves and add to the garlic. Grate 1 tsp of zest from a lemon and add to the herbs and garlic. Using a spoon, mash the herbs into 200g of softened butter, adding 6 tbsp of white breadcrumbs and a grinding of black pepper. Wash and remove the beards from 500g of mussels. Check each one by tapping it firmly on the side of the sink, discarding any whose shells don’t close immediately, or whose shells are broken or chipped. Put the mussels with 100ml of water into a medium-sized saucepan, cover tightly with a lid, then bring to the boil. Let the mussels cook for 2 or 3 minutes until their shells are open. Remove immediately from the heat. Heat an overhead grill. Remove and discard the top, empty shell from each mussel. Place the mussels in their shells on a baking sheet or grill pan, then put a spoonful of the herb butter on top of each. Put the mussels under the hot grill and let the butter melt and start to bubble. Enough for 2 Rather than bang the mussels on the sink to check they are still alive, I prefer to press each one firmly with my thumb and fingers, pushing the top shell sideways. If it is alive, it will resist the pressure. The mussel shells may wobble and spill their herb butter over the baking sheet if you don’t secure them. I place a layer of salt on the sheet to keep them steady as they cook. If there is any herb butter left over, use it later on toast, baked potatoes or over grilled lamb cutlets. The Observer aims to publish recipes for fish rated as sustainable by the Marine Conservation Society’s Good Fish Guide